Ingredients

• 1 cup water
• 2 ounces unsweetened chocolate, chopped
• 1 cup (2 sticks) unsalted butter, room temperature
• 1 1-pound box dark brown sugar
• 3 large eggs
• 1 teaspoon baking soda
• 2 cups all purpose flour, sifted
• 2 ounces unsweetened chocolate, chopped
• 2 tablespoons (1/4 stick) unsalted butter
• 1 1/2 cups powdered sugar
• 1 tablespoon (or more) hot water
• 1/2 teaspoon vanilla

Instructions

1. Preheat oven to 350°F. Lightly grease 13 x 9 x 2-inch baking pan. Combine 1 cup water and chocolate in heavy small saucepan over low heat. Stir until chocolate melts and mixture is smooth. Let cool 10 minutes.
2. Using electric mixer, beat 1 cup butter and dark brown sugar in large bowl until fluffy, about 3 minutes.
3. Add eggs 1 at a time, beating well after each addition (mixture will resemble thick frosting). Stir baking soda into chocolate mixture. Mix flour into butter mixture alternately with chocolate mixture in 2 additions each.
4. Pour batter into prepared pan. Bake until tester inserted into center of cake comes out clean, about 40 minutes. Transfer to rack and cool completely.
5. For frosting: Stir chocolate and butter in heavy small saucepan over low heat until melted and smooth. Remove from heat. Sift powdered sugar into medium bowl. Add chocolate mixture, 1 tablespoon water and vanilla and stir until smooth. Thin to spreading consistency with more water if necessary. Spread frosting over cake. (Can be prepared 1 day ahead. Cover with foil and let stand at room temperature.) Cut into squares and serve.
6. Stir chocolate and butter in heavy small saucepan over low heat until melted and smooth. Remove from heat. Sift powdered sugar into medium bowl. Add chocolate mixture, 1 tablespoon water and vanilla and stir until smooth. Thin to spreading consistency with more water if necessary.
7. Spread frosting over cake. (Can be prepared 1 day ahead. Cover with foil and let stand at room temperature.) Cut into squares and serve.