Bake an angel food cake as directed on package. Cool completely. Remove from pan; place cake upside down. Slice off entire top of cake about 1" down; set aside.
Make cuts down into cake 1" from outer edge and 1" from edge of hole, leaving substantial "walls" on each side.
With a curved knife or spoon, remove cake within cuts, being careful to leave a base of cake 1" thick. Place cake on serving plate.
In chilled bowl, beat 3 cups chilled whipping cream, 1 1/2 cups confectioners' sugar, 3/4 cup cocoa and 1/4 teaspoon salt until stiff. Fold 1/3 cup toasted slivered blanched almonds into half the mixture; spoon into cake cavity.
Press mixture firmly into cavity to avoid holes in cut slices. Replace top of cake; press down gently. Frost cake with remaining whipped cream mixture. Sprinkle 1/3 cup toasted slivered blanched almonds over side and top of cake. Chill at least 4 hours or until set.
STRAWBERRY CREAM WALDORF: Decrease confectioners' sugar to 1/3 cup; omit cocoa and salt. Tint whipped cream mixture with 2 drops red food color; fold 1 package (10 ounces) frozen sliced strawberries, thawed and drained, and 6 large marshmallows, quartered, with the almonds into half the mixture.