Heat oven to 350°F. Spray 15x10x1-inch pan with nonstick cooking spray. Line bottom of pan with waxed paper; spray paper with cooking spray.
Make cake mix as directed on box using water, oil and eggs. Pour batter into sprayed paper-lined pan.
Bake 18 to 20 minutes or until cake springs back when touched lightly in center. Cool cake in pan on wire rack 10 minutes. Invert cake onto wire rack; remove pan and paper. Cool completely, about 15 minutes.
Meanwhile, in medium saucepan, melt chocolate over low heat, stirring constantly. Remove from heat. Stir in almond extract. Cool 10 minutes or until slightly cooled. Genlty stir cooled chocolate 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 pastry bag fitted with star tip. Pipe border around bottom and top of cake. Decorate top of cake with mousse rosettes. Sprinkle top with almonds. Store in refrigerator.