Coat two 13 x 9 x 2-inch baking pans with cooking spray. Line with nonstick foil or waxed paper, extending 2 inches over short ends. Coat waxed paper with cooking spray.

In medium-size bowl, microwave unsweetened chocolate on 100 percent power 1 minute; stir until smooth. In small bowl, microwave white baking chocolate on 100 percent power 1 minute; stir. Then microwave in 15 second intervals, stirring until melted.

Heat oven to 350 degrees F.

In large bowl, on medium speed, beat together butter, sugar, salt and mint extract until light, about 3 minutes. Add eggs, one at a time, beating well after each addition. On low speed, beat in flour until blended (about 5 cups batter). Remove 1-2/3 cups batter and stir into melted unsweetened chocolate. Spread evenly into one of prepared pans.

Into remaining batter, stir melted white chocolate until smooth. Remove 1-2/3 cups of this batter and spread evenly in second pan. Bake both pans in 350 degree F oven until toothpick comes out clean, 11 to 14 minutes. Transfer to wire racks; let cool 10 minutes.

Meanwhile, stir 5 drops green food coloring into remaining 1-2/3 cups batter.

Turn cakes out onto racks; remove foil or waxed paper; let cool.

Wash and dry one pan; coat and line as in Step 1. Pour in green batter; bake as in Step 5.

Place cooled white chocolate layer on a baking sheet. Spread with 1/4 cup mint jelly. Top with chocolate layer; spread with remaining jelly. Top with green layer. Cover with plastic wrap; place a baking sheet on top. Weight with cans. Refrigerate 2 hours.

In small bowl, microwave chips and heavy cream on 100 percent power for 1 minute; stir until melted. Remove wrap from cake; spread top with chocolate-cream mixture. Refrigerate, uncovered, until top is set. Trim edges. Cut cake crosswise into 1-inch-wide strips; cut each crosswise into quarters. Makes 4 dozen bars.