Don't be tempted to use Dutch process cocoa powder in the crust or the pastry will be tough. However, it has a mild chocolate flavor and dark brown color perfect for the filling.
4.5 ounces all-purpose flour (about 1 cup)
2 tablespoons sugar
2 tablespoons unsweetened cocoa
1/4 teaspoon salt
2 1/2 tablespoons vegetable shortening
2 tablespoons chilled butter, cut into small pieces
1/4 cup ice water
3 tablespoons butter
1 ounce semisweet chocolate chips
1 teaspoon vanilla extract
4 large egg whites
1 cup sugar
3.4 ounces all-purpose flour (about 3/4 cup)
1/3 cup Dutch process cocoa
1/2 teaspoon baking powder
Dash of salt
How to Make It
To prepare crust, weigh or lightly spoon 5 ounces (about 1 cup) flour into a dry measuring cup; level with a knife. Combine 5 ounces flour, 2 tablespoons sugar, 2 tablespoons unsweetened cocoa, and 1/4 teaspoon salt in a food processor; pulse 2 times or until blended. Add shortening and chilled butter; pulse 6 times or until mixture resembles coarse meal. With processor on, slowly pour ice water through food chute, processing just until blended (do not allow dough to form a ball); remove from bowl. Gently press mixture into a 4-inch circle; wrap in plastic wrap. Chill 30 minutes.
Preheat oven to 350°.
Unwrap and place chilled dough on plastic wrap. Lightly sprinkle dough with flour; roll to a 10-inch circle. Fit dough, plastic wrap side up, into a 9-inch pie plate coated with cooking spray. Remove remaining plastic wrap. Fold edges under, and flute.
To prepare filling, place 3 tablespoons butter and chocolate in a microwave-safe bowl. Microwave at HIGH 30 seconds or until butter and chocolate melt, stirring well to combine. Place vanilla and egg whites in a bowl; beat with a mixer at medium speed until foamy. Gradually add 1 cup sugar; beat until soft peaks form (about 2 minutes). Gently fold melted chocolate mixture into egg white mixture.
Weigh or lightly spoon 4 ounces flour and 1/3 cup Dutch process cocoa into dry measuring cups; level with a knife. Combine flour, cocoa, baking powder, and dash of salt in a small bowl, stirring with a whisk. Fold flour mixture into egg white mixture. Pour mixture into prepared crust. Bake at 350° for 40 minutes or until a wooden pick inserted in center comes out clean. Cool on a wire rack.