For Brownie:Preheat oven to 350 degrees F. Line an 8-by-8-inch baking pan with lightly buttered parchment paper.Melt butter and vegetable oil in a small saucepan or in the microwave on medium heat. Pour into mixing bowl with sugar and stir until paste-like. Add eggs, one at a time, whisking until completely incorporated before adding the next. Mix in vanilla.Whisk together flour, cocoa powder, baking powder and salt. Add to bowl with butter mixture and stir until almost fully incorporated and just a few streaks of dry ingredients remain. Add milk and stir until smooth.Pour into prepared baking pan. Bake for 25 to 30 minutes or until a toothpick inserted near the center comes out mostly clean with a few moist crumbs attached. Place pan on a wire rack and let cool completely.When brownie is cool, lift out of pan using parchment paper as handles. Cut into a 6-inch circle (about 1-inch smaller than the size of your final cake). Spread a thin layer of Nutella on top, then place in freezer for at least 30 minutes or overnight.For Mousse/Assembly:Cut gelatin sheets into 1-inch strips and submerge in cool water for at least 5 minutes to soften. If using powdered gelatin, sprinkle 2 teaspoons over 2 tablespoons of cool water to soften.Melt chocolate and nutella in a double boiler or in the microwave on short 30 second intervals at 50% power.Heat 1/2 cup cream in a small saucepan over medium heat until it just begins to bubble around the edges. Remove from heat and add gelatin (if using leaf gelatin, squeeze out as much water as possible first) and whisk until completely melted. Pour over chocolate and Nutella. Gently whisk, starting in the center and working in small concentric circles, until a smooth and shiny emulsion forms, then gradually whisk in larger circles until all the cream is completely incorporated and mixture is smooth. Stir in a pinch of sea salt.In a clean bowl or the bowl of a stand mixer, beat remaining 1 cup cream until it holds soft peaks. Add 1/3 of cream to the chocolate mixture and fold to lighten, then add the entire chocolate mixture to the bowl with the cream and fold until fully incorporated.To assemble, place your frozen brownie round into the bottom of an 8-by-2-inch deep silicone cake pan, Nutella side up. Pour mousse over top of brownie. Place silicone cake pan on a hard, flat surface, and firmly tap it a few times against the counter to remove any air bubbles and even out the top surface. If necessary, use an offset spatula to spread the mousse into as smooth and even a surface as possible.Place entire cake into freezer and let freeze until completely firm, at least 6 hours or overnight if possible. Remove from silicone pan. Smooth out any rough spots with an offset spatula if necessary. Place, brownie side down, on a cake board or other solid surface and put back in the freezer until ready to glaze.For Mirror Glaze:(Mirror glaze can be made up to 5 days ahead of time; store in an airtight container in the refrigerator and rewarm to ideal temperature before using. This recipe makes enough glaze for two 8-inch cakes).Cut gelatin into 1-inch strips and submerge in cool water for at least 5 minutes to soften. Place milk chocolate in a heat-proof bowl and set aside.Combine sugar, corn syrup, and water in a saucepan and bring to a boil over medium heat until sugar is completely dissolved and mixture is clear.Remove from heat, then add gelatin (squeeze out as much water as possible first) and whisk until dissolved. Whisk in sweetened condensed milk. Pour hot mixture over chopped chocolate and let sit for 30 seconds, then stir until chocolate is completely melted and mixture is smooth (you can also use an immersion blender if you have one to expedite this process, just try not to mix air bubbles into the glaze if possible).Let glaze cool, stirring occasionally, until glaze reaches approximately 95 degrees F on an instant-read thermometer. The glaze needs to be the correct temperature or it will not form a thick enough layer on the outside of the cakes.When the glaze reaches about 94 degrees, remove your cake from freezer and place on a cookie sheet, elevated off the sheet using something slightly smaller in diameter than the cake itself so the glaze can drip off (I used the same 6-inch cake ring I used to cut out my brownie). You can always rewarm the glaze if it gets too cool (do so gently over a double boiler or in very short intervals in the microwave).Pour glaze generously over cake, taking care to ensure there are no bare spots left. Allow the excess glaze to drip off for 5 or 10 minutes, then gently scrape the edges to remove any remaining drops or built up glaze. If desired, splatter with edible gold luster and press some chocolate sprinkles around the bottom edge. Carefully transfer cake to cake stand or serving plate and refrigerate until ready to serve.Let sit at room temperature for 15 minutes before serving. For perfect slices, dip a large knife into hot water to warm it before slicing, and wipe off the blade completely between each cut. Glazed cake will keep uncovered in the refrigerator for up to 5 days.