HEAT oven to 350°F.
SPRAY 2 (9-inch) round pans with cooking spray; dust with cocoa powder. Coarsely chop 7 cookies; set aside. Finely crush remaining cookies; place in medium bowl. Add flour, baking powder and soda; mix well.
BEAT egg whites in small bowl with mixer on high speed until stiff peaks form; set aside. Beat granulated sugar, 1/3 cup butter and egg yolks in large bowl with mixer until well blended. Add flour mixture, water, sour cream, melted chocolate and 1 tsp. vanilla; beat on medium speed 1 min. Gently stir in egg whites until well blended. Pour into prepared pans.
BAKE 25 min. or until toothpick inserted in centers comes out clean. Cool in pans 10 min. Remove from pans to wire racks; cool completely.
BEAT 1 cup powdered sugar with remaining butter and vanilla in large bowl with mixer until well blended. Add remaining powdered sugar alternately with milk, beating well after each addition. Reserve 1/2 cup frosting.
STACK cake layers on plate, filling with remaining frosting and chopped cookies. Spread top with Chocolate Glaze. Let stand 10 min. Decorate with reserved frosting. Keep refrigerated.