Heat oven to 350 degrees F. Spray 15x10x1 inch pan with cooking spray. Line bottom with waxed paper, spray paper. Prepare cake mix as directed on box using water, oil and eggs. Pour batter into pan.
Bake 18 to 20 minutes or until cake springs back when touched lightly in center. Cool cake in pan on cooling rack 10 minutes. Invert cake onto cooling rack; remove pan and paper. Cool completely, about 15 minutes.
Meanwhile, melt chocolate and 1/2 cup whipping cream in medium saucepan over low heat, stirring constantly. Remove from heat. Stir in almond extract. Cool slightly, about 10 minutes. Fold cooled chocolate mixture into whipped cream.
Trim edges of cake. Cut cake lengthwise into 2 long layers. Place 1 layer on serving platter. Spread with 1/3 of mousse. Repeat with remaining cake layer and 1/3 of mousse. Place remaining 1/3 of mousse in decorating bag fitted with star tip. Pipe border around bottom and top of cake. Decorate top of cake with mousse rosettes. Sprinkle almonds over top. Store in refrigerator. (If you have a lot of mousse left over, you can ice the sides of the cake)