12 servings (serving size: 1 wedge)
Preheat oven to 300°.
Coat bottom of a 9-inch springform pan with cooking spray. Set aside.
Combine 1/2 cup granulated sugar, 1/2 cup brown sugar, water, and espresso in a large saucepan; stir well and bring to a boil. Remove from heat; add cocoa, salt, and chocolates, stirring with a whisk until chocolate melts. Stir in Kahlúa and egg yolks. Stir in flour; cool to room temperature. Set aside.
Beat egg whites and cream of tartar at high speed of a mixer until foamy. Add 1/3 cup granulated sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently fold one-fourth of egg white mixture into chocolate mixture; repeat procedure with remaining egg white mixture, one-fourth at a time. Spoon into prepared pan. Bake at 300° for 1 hour or until a wooden pick inserted in center comes out almost clean. Cool completely on wire rack. Remove sides from pan; sift powdered sugar over cake. Garnish with raspberries and chocolate curls, if desired.
Note: A substitution of 1/4 cup all-purpose flour may be used in place of 1/3 cup cake flour.
Cake flour is a fine-textured, soft wheat flour with a high starch content, and usually comes in a box rather than a bag. It can be found with the cake mixes at the supermarket.