Prepare Base: Press crushed biscuits firmly into the bottom of a 9×9 inch dish.
Slice Cake: Cut sponge cake horizontally into thin, even layers.
Whip Cream: Beat 2 cups cold heavy cream with powdered sugar and vanilla extract until soft peaks form, about 3-4 minutes.
Make Ganache: Heat 1/2 cup heavy cream until simmering. Remove from heat and add chopped chocolate and butter. Let sit 2 minutes, then stir until smooth. Cool slightly.
Layer Dessert: Over biscuit base, add a layer of sponge cake. Spread whipped cream over cake, then drizzle with ganache. Repeat layers, ending with whipped cream and ganache on top.
Chill: Cover and refrigerate for at least 2 hours before serving.
Serve: Slice with a hot knife, wiping between cuts. Serve chilled.