Preheat oven to 350F. Line bottom of 8" springform with foil.

Melt chocolate and butter in microwave. Cool to room temperature. Add yolks, kahlua, and vanilla to COOLED chocolate mixture and whisk until blended.

Beat egg whites until frothy. Gradually add sugar and continue beating until whites are stiff but not dry.

Add 1/3 egg whites to chocolate mixture and whisk lightly to combine. Add remaining egg whites and whisk until blended.

Spread chocolate mixture into lined cake pan.

Bake cake base 18-20 minutes (cake will rise and then fall). Cool to room temperature.

Melt chocolate and butter in microwave. Cool to room temperature. Add yolks, Kahlua, and vanilla to COOLED chocolate mixture and whisk until blended.

Beat egg whites until frothy. Gradually add sugar and continue beating until whites are stiff but not dry.

Add 1/3 egg whites to chocolate mixture and whisk lightly to combine. Add remaining egg whites and whisk until blended.

Whip cream and fold into COOLED chocolate mixture.

While base is still in the pan, spread mousse on top of base. Refrigerate until firm.

Remove cake from pan and place on cake board, mousse side up. Top with whipped cream and pipe rosettes around top border. Decorate center of cake with chocolate curls. Dust lightly with powdered sugar.