Lemony White Cake with Milk Chocolate Ganache

2 1/4 cups cake flour
3 teaspoons baking powder
1/2 teaspoon salt
1/2 cup (1 stick) unsalted butter, softened
1 1/2 cups granulated sugar
2 teaspoons finely grated lemon zest
1/2 teaspoon lemon extract
1/2 cup egg whites (from about
4 large eggs)
1 1/4 cups milk

Milk Chocolate Ganache
Zest of 2 lemons removed in long strips with a vegetable peeler
2 cups heavy whipping cream
4 tablespoons (1/2 stick) unsalted butter, softened
20 ounces milk chocolate, cut into 1/4-inch pieces
4 ounces bittersweet chocolate, cut into 1/4-inch pieces

Butter two 9-inch round pans, 1 1/2 to 2 inches deep, and line with buttered parchment or wax paper. Set a rack at the middle level of the oven and preheat to 350 degrees F.

Sift the cake flour, baking powder and salt onto a piece of parchment or wax paper and set aside.

Using an electric mixer set at medium speed, beat the butter and sugar until light, about 3 minutes. Beat in the lemon zest and extract.

In a bowl, whisk together the egg whites and milk.

Add a third of the flour mixture to the butter and sugar mixture and beat until smooth. Scrape down bowl and beaters. Beat in half the milk and egg white mixture until incorporated, then beat in another third of the flour mixture. Scrape bowl and beaters. Beat in remaining liquid until absorbed, followed by remaining flour mixture. Scrape well after each addition.

Divide batter between prepared pans and smooth top evenly. Bake for about 30 to 35 minutes, until well risen and a wooden pick inserted in the center emerges clean. Cool layers in pans for 5 minutes, then invert to racks to cool. Peel off paper. If prepared in advance, double-wrap layers in plastic wrap and chill for up to several days or freeze.

Milk Chocolate Ganache: Place the pieces of zest in a saucepan and add the cream. Place over low heat and bring to a simmer. Remove from heat and allow to steep about 5 minutes. Remove zests from cream with a slotted spoon and discard them. Add butter to the cream and bring to a boil over low heat. Remove from heat and add chocolates. Shake pan to submerge chocolate and allow to stand 5 minutes. Whisk smooth, then cool to room temperature. Ganache will thicken to spreading consistency.

To finish, put one layer right side up on a platter or cardboard. Place ganache in mixer bowl and beat until light, about 20 seconds. Using an offset spatula, spread the layer with about a third of the ganache. Place the other cake layer upside down on the ganache, so that the smooth bottom of the cake layer is uppermost. Spread the top and sides of the cake evenly with most of the remaining ganache, but save about 1/2 to 1/3 cup to pipe decorations on top of the cake.

After smoothing the outside of cake, use a pastry bag fitted with a medium star tube and pipe a series of rosettes around the top rim of the cake.

Storage: Keep the cake at a cool room temperature before serving. Keep leftovers under a cake dome at a cool room temperature or covered with plastic wrap in the refrigerator.