Preheat oven to 325°F. Butter and flour two 9-inch cake pans, lining the bottoms with parchment paper.

Melt bittersweet chocolate in a double boiler and cool to lukewarm.

Whisk flour, baking soda, and salt in a bowl.

Cream together sugar, butter, and vanilla in a separate bowl using an electric mixer.

Beat in eggs one at a time, then gradually beat in the cooled chocolate.

Alternate adding dry ingredients and sour cream to the batter, ending with dry ingredients.

Stir in creme de cacao and coffee.

Divide batter evenly between the prepared pans and bake for 35 minutes, or until a toothpick comes out clean.

Cool cakes in pans for 10 minutes, then invert onto cardboard rounds or tart pan bottoms to cool completely.

Melt milk chocolate in a double boiler.

Whisk sugar, egg yolks, water, and corn syrup in a metal bowl. Add 1/4 cup of butter.

Set the bowl over simmering water and whisk until the mixture reaches 170°F. Remove from heat.

Beat the mixture with an electric mixer until completely cool and thick.

Gradually beat in the remaining butter, then the lukewarm melted chocolate.

Bring half-and-half, sugar, and espresso powder to a simmer in a saucepan.

Whisk egg yolks and sugar in a bowl. Gradually whisk in the hot half-and-half mixture.

Return the mixture to the saucepan and stir until it reaches 160°F.

Pour into a bowl and add softened gelatin, stirring until dissolved.

Beat until cool with an electric mixer.

Whip cream and vanilla in a separate bowl until stiff peaks form. Fold into the coffee mixture.

Cut each cake layer horizontally in half.

Place one cake layer in a springform pan, cover with buttercream, then another cake layer.

Top with mousse and another cake layer. Refrigerate for 1 hour.

Spread buttercream over the third layer and top with the final cake layer. Cover and refrigerate overnight.

For the glaze, stir sugar and water in a saucepan until sugar dissolves. Bring to a boil.

Whisk in cocoa, remove from heat, and add chocolate. Whisk until smooth. Cool for 2 hours.

Loosen cake from pan, release sides, and transfer to a rack over a baking sheet.

Pour glaze over the cake, spreading with a spatula as needed. Refrigerate for 2 hours before serving.