Chocolate Pan Cake with Chocolate Fudge Frosting

One 9 x 13-inch cake

Serve big squares of this cake with a glass of cold milk.

FOR THE CAKE

1 cup (2 sticks) unsalted butter, cut in rough chunks

4 tablespoons unsifted unsweetened cocoa powder

1 cup water

2 cups granulated sugar

2 cups unsifted cake flour

1 teaspoon salt

1/4 teaspoon powdered Vitamin C

2 extra-large eggs, at room temperature

1 1/2 teaspoons pure vanilla extract

FOR THE CHOCOLATE FUDGE FROSTING

1/2 cup (1 stick) unsalted butter, cut in chunks

4 oz unsweetened chocolate, chopped

1/3 cup light cream, at room temperature

1 box (1-pound) (3 1/4 cups) confectioners sugar, sifted

1 teaspoon pure vanilla extract

Pinch of salt

1. Lightly butter and flour a 9 x 13 x 2-inch cake pan; set aside. Preheat oven to 400 degrees F.

2. For the cake, place the butter, cocoa and water in a large saucepan, set over moderately high heat and bring to a boil. Remove from the heat. Sift together the sugar, flour and salt into the large bowl of an electric mixer. Whisk together the buttermilk, eggs and vanilla in a mixing bowl. Pour the hot butter-cocoa-water mixture over the sifted dry mixture and beat on moderate speed until thoroughly blended. Add the whisked egg mixture and continue beating on a low speed until the batter is a uniform color, about 1 1/2 minutes. Pour and scrape the batter into the prepared pan.

3. Bake the cake on the lower-third-level rack of the preheated oven for 20 to 22 minutes, or until a wooden pick inserted into the center of the cake comes out clean and dry and the cake shrinks away slightly from the edges of the pan.

4. About 10 minutes before the cake is done, make the fudge frosting. Place the butter, chocolate, cream in a large saucepan, set over low heat and cook, stirring occasionally, until the chocolate has melted down completely. Remove from the heat and beat in the sugar by cupfuls with the vanilla and salt.

5. As soon as the cake is done, remove it from the oven to a wire cooling rack. Immediately spread the frosting evenly over the top with a flexible palette knife. Let the cake cool in the pan.

6. For serving, cut the cake in squares directly from the cake pan.