Dark Chocolate Souffle Cake

Yield
12 servings (serving size: 1 wedge)
Ingredients
Cooking spray
1/2 cup granulated sugar
1/2 cup packed dark brown sugar
3/4 cup water
1 tablespoon instant espresso or 2 tablespoons instant coffee granules
2/3 cup Dutch process or unsweetened cocoa
1/4 teaspoon salt
2 ounces semisweet chocolate, chopped
2 ounces unsweetened chocolate, chopped
2 tablespoons Kahlúa (coffee-flavored liqueur)
3 large egg yolks
1/3 cup sifted cake flour (such as Swan's Down)
6 large egg whites (at room temperature)
1/4 teaspoon cream of tartar
1/3 cup granulated sugar
1 tablespoon powdered sugar
1/4 cup raspberries (optional)
Chocolate curls (optional)
Preparation
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.