6 ounces unsweetened chocolate, coarsely chopped
3/4 cup unsalted butter
1 1/4 cups all-purpose flour
1 teaspoon baking powder
1/4 teaspoon salt
1 3/4 cups granulated sugar
4 eggs
2 teaspoons vanilla extract

Preheat oven to 350 degrees F. Coat a 15 x 10-inch jellyroll pan with a nonstick cooking spray.

Melt chocolate and butter in bowl over simmering water in saucepan. Stir until smooth. Stir together flour, baking powder and salt in small bowl. Stir together sugar, eggs, vanilla extract and chocolate-butter mixture in second bowl. Stir in flour mixture. Spread in prepared pan. Bake for 15 to 20 minutes or until dry-looking on top and firm to the touch. Meanwhile, prepare frosting.

Frosting
2 cups granulated sugar
2/3 cup milk
1/2 cup unsalted butter
4 ounces unsweetened chocolate, coarsely chopped
2 tablespoons corn syrup
1 1/2 teaspoons vanilla extract

Combine sugar, milk, butter, chocolate and corn syrup in medium-size saucepan. Bring to boiling over medium-high heat; cook 1 minute. Remove from heat. Stir in vanilla extract. Cool to room temperature.

Beat with a mixer until spreading consistency. Cool in pan on wire rack. Spread evenly with frosting. Let stand until set, about 30 minutes.