Bake angel food cake mix 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 about 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 tsp. salt until stiff. fold 1/3 cup toasted slivered blanched almonds into half of 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 mixture. Sprinkle 1/3 cup toasted slivered blanched almonds over side and tip of cake. Chill at least 4 hours or until set.

12 to 16 servings.

Strawberry Cream Waldorf:
Decrease confectioners' sugar to 1/3 cup; omit cocoa and salt. Tint whipped cream mixture with 2 drops red food coloring; fold 1 pkg (10oz) frozen sliced strawberries, sliced and drained, and 6 large marshmallows, quartered, with the almonds into half the mixture.