Active Time

　　1 hr

　　Total Time

　　2 1/2 hr (includes cooling)

　　For more information about melting chocolate, see Meltdown.

　　

Ingredients

Makes 8 to 10 servings

　　

For cake

2 cups all-purpose flour

　　2/3 cup unsweetened cocoa powder (not Dutch-process)

　　1 1/2 teaspoons baking soda

　　1/2 teaspoon salt

　　2 sticks (1 cup) unsalted butter, softened

　　1 cup packed dark brown sugar

　　3/4 cup granulated sugar

　　4 large eggs at room temperature for 30 minutes

　　2 oz unsweetened chocolate, melted and cooled

　　1 1/2 teaspoons vanilla

　　1 1/2 cups well-shaken buttermilk

　　

For frosting

2/3 cup whole milk

　　3 large egg yolks

　　1 tablespoon plus 1 teaspoon all-purpose flour

　　1 1/3 cups confectioners sugar

　　1 teaspoon vanilla

　　3 sticks (1 1/2 cups) unsalted butter, cut into tablespoon pieces and softened

　　8 oz milk chocolate, melted and cooled

　　2 oz unsweetened chocolate, melted and cooled

　　

Make cake:

Step 1

Put oven rack in middle position and preheat oven to 350°F. Butter 2 (9- by 2-inch) round cake pans and line bottom of each with a round of parchment or wax paper. Butter paper and dust pans with flour, knocking out excess.

　　

Step 2

Sift together flour, cocoa, baking soda, and salt into a small bowl. Beat together butter and sugars in a large bowl using an electric mixer at medium-high speed until light and fluffy, 3 to 4 minutes in a standing mixer or 4 to 5 minutes with a handheld. Add eggs 1 at a time, beating well after each addition. Add chocolate and vanilla and beat until just combined. Reduce speed to low and add flour mixture and buttermilk alternately in 3 batches, beginning and ending with flour mixture and mixing until just combined.

　　

Step 3

Divide batter between cake pans, spreading evenly, and bake until a wooden pick or skewer inserted in center of each cake layer comes out clean, 25 to 35 minutes.

　　

Step 4

Cool cake layers in pans on racks 10 minutes. Run a thin knife around edge of each layer, then invert onto racks. Peel off paper and cool layers completely.

　　

Make frosting:

Step 5

Heat milk in a 1- to 1 1/2-quart heavy saucepan over moderate heat until hot. Whisk together yolks, flour, 1/3 cup confectioners sugar, and a pinch of salt in a bowl, then add hot milk in a stream, whisking. Transfer custard to saucepan and bring to a boil over moderate heat, whisking. Reduce heat and simmer, whisking, 2 minutes (mixture will be very thick), then transfer to a large bowl. Cover surface of custard with a buttered round of wax paper and cool completely, about 45 minutes.

　　

Step 6

Add vanilla and remaining cup confectioners sugar to custard and beat with cleaned beaters at moderate speed until combined well, then increase speed to medium-high and beat in butter, 2 tablespoons at a time, until smooth. Add chocolates and beat until combined well.

　　

Frost cake:

Step 7

Halve each cake layer horizontally using a long serrated knife. Layer cake, using a heaping 1/2 cup frosting between each layer, then frost top and sides with remaining frosting.

　　Cooks' notes:

　　·Cake layers can be made (but not halved) 1 day ahead and kept, wrapped well in plastic wrap, at room temperature. ·Frosting can be made 1 day ahead and chilled, covered. Bring to room temperature (do not use a microwave) and beat with an electric mixer before using.