Preheat oven to 350°F (176°C) and prepare an 8-inch cake pan with parchment paper on the bottom; grease the sides.

In a large mixing bowl, cream together butter and sugar for 3–4 minutes until light and fluffy.

Mix in sour cream and vanilla. Add egg whites one at a time, blending well after each addition.

In a separate bowl, whisk flour, cocoa, baking powder, and salt. In a measuring cup, combine milk and water.

Add half the dry mix to the batter, then the milk mixture, then the remaining dry mix. Mix until smooth.

Pour into pan and bake for 19–21 minutes, or until a toothpick comes out with a few crumbs. Cool completely.

Prepare the pan for layering by lining a springform pan with parchment extending 2 inches above the rim. Place the cooled cake inside.

For the fudge, place chocolate chips, corn syrup, and vanilla in a bowl. Heat cream until just boiling, pour over chocolate, cover for 3–5 minutes, then whisk smooth.

Spread fudge over the cake and refrigerate 1 hour.

For the chocolate mousse, whisk egg yolks and sugar until pale. Warm part of the cream, whisk into yolks, then stir in melted chocolate. Cool slightly. Whip remaining cream with powdered sugar until fluffy, fold gently into chocolate base. Spread over fudge, chill 30 minutes.

For the white chocolate mousse, bloom gelatin in water. Melt white chocolate, then fold into whipped cream sweetened with powdered sugar. Stir in dissolved gelatin. Spread over dark mousse layer.

Refrigerate the cake for at least 5–6 hours, preferably overnight.

Slice with a hot knife for clean layers and garnish with cocoa powder or chocolate curls before serving.