Step 1

Preheat oven to 350°F. Grease three 8-inch round cake pans and line bottom with parchment paper. Set aside.

Step 2

In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.

Step 3

In the bowl of a stand mixer fitted with a paddle attachment (or using an electric hand mixer), cream together butter and sugar until smooth. Add eggs and mix on medium speed until well incorporated. Add vanilla and sour cream and mix until combined. Scrape down bowl as needed to ensure thorough mixing.

Step 4

Add flour mixture in three additions, alternating with milk. Begin and finish with flour mixture. Mix until just combined and there are no longer any dry streaks of flour.*

Step 5

Divide batter evenly among the three prepared cake pans. Level batter using a mini offset spatula or the back of a spoon. Bake for 28-30 minutes until a toothpick inserted in center of cake comes out clean. Cool cakes in pan for 5 minutes. Then, run a mini offset spatula around the cake and unmold. Cool cake to room temperature on wire rack.

Step 6

Mix together passion fruit pulp and simple syrup. Set aside until ready to use.

Step 7

In the bowl of a stand mixer, whisk together egg whites and sugar. Place over a pot of simmering water to create a bain-marie or double boiler. Make sure the simmering water does not touch the bottom of the bowl. Whisk mixture until it reaches a temperature of 160°F.*Alternatively, if you don't have a thermometer, heat the mixture until all the sugar has melted. Test this by dipping your pointer finger into the mixture and rubbing it against your thumb. The mixture should feel smooth. You should not be able to feel the granules of sugar.

Step 8

Return bowl to stand mixer. Using the whisk attachment, beat mixture at medium speed until it has cooled to room temperature (about 75-80°F) and meringue has started to form.

Step 9

Reduce to low speed. Add butter one tablespoon at a time. Add salt. Increase speed to medium-high and continue to beat until buttercream is light and fluffy.

Step 10

Fit a piping bag (decorating bag) with a small round tip (wilton #12). Fill bag with about ½ cup of buttercream. Set aside.

Step 11

Level cake layers as necessary. Place one cake layer on serving platter or cake turntable. Brush or drizzle passion fruit syrup over cake. Spread a thin layer of buttercream (about 6 Tbsp) over cake layer using a small offset spatula.

Step 12

Use the filled piping bag, pipe a double border of buttercream along the perimeter of cake round. This will serve as barrier when curd is added.

Step 13

Add ⅓ cup of passion fruit curd to center of cake. Spread to fill within the buttercream boundaries. Place second cake layer on top. Repeat soaking cake, adding buttercream, and curd.

Step 14

Place final third layer on top. Soak cake layer with syrup. Spread buttercream all around the cake to create a "crumb coat" layer to seal in any crumbs. Chill cake in the fridge for 20 minutes to set frosting.

Step 15

If desired, coat chilled cake with additional buttercream. Use a small leaf piping tip (wilton #to pipe decorative border on top of cake.

Step 16

Keep cake chilled until ready to serve. Rest cake at room temperature for about 30 minutes before slicing and serving to soften buttercream frosting.