Prepare the chocolate base: Melt 6 oz of dark chocolate with butter in a double boiler, stirring until completely smooth and glossy. Let cool slightly.
Whip the egg whites: Beat egg whites until soft peaks form, then gradually add half the sugar. Continue beating until stiff, glossy peaks develop.
Combine wet ingredients: Whisk egg yolks with remaining sugar until pale and fluffy. Fold in the cooled chocolate mixture, vanilla, and espresso powder.
Fold everything together: Gently fold the chocolate mixture into the egg whites in three additions. Sift cocoa powder over the batter and fold until just combined.
Bake the cake: Pour batter into a greased 9-inch springform pan and bake at 350°F for 25–30 minutes, until the center is just set but still slightly jiggly.
Create the mousse layer: Heat remaining cream and pour over chopped chocolate. Let sit 2 minutes, then whisk until smooth. Once cooled, whip until light and fluffy.
Assemble your masterpiece: Spread the chocolate mousse over the cooled cake base. Refrigerate for at least 4 hours or overnight to set.
