Prepare Cake Layers:Preheat oven to 350°F. Butter and line a quarter sheet pan with parchment paper or a silicone baking mat.Combine the butter and sugars in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes. Scrape down the sides of the bowl, add the eggs, and mix on medium-high again for 2 to 3 minutes. Scrape down the sides of the bowl once more.On low speed, stream in the buttermilk, oil, and vanilla. Increase the mixer speed to medium-high and paddle for 4 to 6 minutes, until the mixture is practically white, twice the size of your original fluffy butter-and-sugar mixture, and completely homogeneous. Don’t rush the process. You’re basically forcing too much liquid into an already fatty mixture that doesn’t want to make room for the liquid. Stop the mixer and scrape down the sides of the bowl.On very low speed, add the cake flour, baking powder, and salt. Mix for 45 to 60 seconds, just until your batter comes together and any remnants of dry ingredients have been incorporated. Scrape down the sides of the bowl. If you see any lumps of cake flour in there while you’re scraping, mix for another 45 seconds.Spread the cake batter in an even layer in the pan. Give the bottom of your sheet pan a tap on the countertop to even out the layer. Sprinkle the chocolate chips evenly over the cake batter.Bake for 30 to 35 minutes or until puffed and doubled in size and the center is set. When gently poked with your finger, the cake should bounce back slightly.Remove from oven and cool on a wire rack. Stored in the fridge or freezer, wrapped in plastic wrap, for up to 5 days.Prepare Passionfruit Curd:Put the passion fruit puree and sugar in a blender and blend until the sugar granules have dissolved. Add the eggs and blend on low until you have a bright orange-yellow mixture. Transfer the contents of the blender to a medium pot or saucepan. Clean the blender canister.Sprinkle the gelatin over 2 tablespoons of cold water in a small bowl to bloom. Set aside for 5 minutes.Heat the passion fruit mixture over low heat, whisking regularly. As it heats up, it will begin to thicken; keep a close eye on it. Once the mixture boils, remove it from the stove and transfer it to the blender. Whisk a few spoonfuls of the hot passionfruit mixture to the bloomed gelatin until softened and smooth, then add to blender along with butter and salt. Blend until the mixture is thick, shiny, and smooth.Transfer the mixture to a heatproof container. Cover with a lid or plastic wrap (pressed down to touch the surface of the curd) and refrigerate until the curd has cooled completely, at least 30 minutes. The curd can be refrigerated for up to 1 week until ready to use; do not freeze.Prepare Chocolate Crumble:Preheat oven to 300°F. Line a baking sheet with parchment paper or a silicone baking mat.Combine the flour, cornstarch, sugar, cocoa powder, and salt in the bowl of a stand mixer fitted with the paddle attachment. Mix on low speed until combined.Add the butter and mix on low speed until it starts to come together in small clusters.Spread the clusters on prepared baking sheet, breaking up into pea-sized pieces as needed. Bake for 20 minutes, breaking them up occasionally. The crumbs should still be slightly moist to the touch at that point; they will dry and harden as they cool.Let the crumbs cool completely before using. Store in an airtight container for up to 1 week at room temperature or 1 month in the fridge or freezer.Prepare Coffee Buttercream:Combine the butter and confectioners’ sugar in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes, until fluffy and pale yellow.Meanwhile, make a quick coffee milk: whisk together the milk, instant coffee, and salt in a small bowl.Scrape down the sides of the bowl with a spatula. On low speed, gradually stream in the coffee milk. The butter mixture will clump up and separate upon contact with the coffee milk. If it does, turn up the mixer speed and mix until the milk is incorporated before adding more. This WILL take a good deal of time (more than you think), just keep on mixing and it will come together eventually into a light and fluffy frosting. Use immediately.To Assemble:Put a piece of parchment or a Silpat on the counter. Invert the cake onto it and peel off the parchment from the bottom of the cake. Use a 6-inch cake ring to stamp out 2 circles from the cake. These are your top 2 cake layers. Cut out two half-circles (as large as you can) from the remaining cake. These, along with any leftover scraps, will come together to make the bottom layer of the cake.Clean the cake ring and place it in the center of a sheet pan lined with clean parchment or a Silpat. Line the inside of the cake ring with 1 strip of acetate.Piece together the two half circles and additional cake scraps as needed to form a flat even layer, using the back of your hand to tamp the scraps together. Brush with a bit of extra passion fruit puree.Use the back of a spoon to spread half of the passion fruit curd in an even layer over the cake.Sprinkle one third of the chocolate crumbs evenly over the passion fruit curd. Use the back of your hand to press them in place.Use the back of a spoon to spread one-third of the coffee frosting as evenly as possible over the crumbs.With your index finger, gently tuck the second strip of acetate between the cake ring and the top ¼ inch of the first strip of acetate, so that you have a clear ring of acetate 5 to 6 inches tall – high enough to support the height of the finished cake. Set a cake round on top of the frosting, and repeat layers of passionfruit puree, curd, crumbs, and buttercream.Nestle the remaining cake round into the frosting. Cover the top of the cake with the remaining frosting. Garnish the frosting with the remaining chocolate crumbs and a few chocolate chips as well, if desired.Place the sheet pan in the freezer and freeze for a minimum of 12 hours to set the cake and filling, or up to 2 weeks.At least 3 hours before you are ready to serve the cake, pull the sheet pan out of the freezer and gently pop the cake out of the cake ring. Carefully peel off the acetate, and place the cake on a platter or cake stand. Let sit in the fridge for a minimum of 3 hours to thaw before slicing and serving. The finished cake can be refrigerated, wrapped in plastic wrap, for up to 5 days.