Vanilla custard filling:
11/2 cups milk, divided
3 tablespoons cornstarch
4 large egg yolks
1/8 teaspoon salt
1/2 cup granulated sugar
1 tablespoon unsalted butter, softened
11/2 teaspoons vanilla extract
2 cups all-purpose flour
21/4 teaspoons baking powder
1/2 teaspoon salt
11/2 cups granulated sugar
Grated zest from 1 large navel orange, about 1 tablespoon
Grated zest from 1 medium lemon, about 2 teaspoons
1/2 vanilla bean, split lengthwise
12 tablespoons (11/2 sticks) unsalted butter, softened
3 large eggs, at room temperature
3/4 cup buttermilk
1 cup orange marmalade
1 tablespoon orange flavored liqueur or orange juice
Milk chocolate frosting:
9 ounces milk chocolate (40 or 41% cocoa solids), finely chopped
3 ounces semisweet chocolate
(53 - 62%), finely chopped
2 tablespoons (1/4 stick) unsalted butter, cut into 1/2" chunks
1 cup sour cream
1 teaspoon vanilla
1. Make custard filling: In medium bowl, combine 1/4 cup milk and cornstarch and whisk until smooth. Add yolks and salt and whisk until blended.
2. Meanwhile, in medium-sized heavy saucepan, combine remaining milk and sugar. Cook over medium-low heat, stirring occasionally, until sugar is dissolved. Raise heat to medium-high and bring to boil. Remove pan from heat and whisk half of milk mixture into yolk mixture. Return yolk mixture to pan and bring back to boil. Boil gently for 1 minute, whisking constantly.
3. Remove pan from heat and whisk in butter until melted. Whisk in vanilla. Quickly strain custard through fine-mesh sieve into medium bowl. Press piece of plastic wrap directly onto surface of custard. Let cool to room temperature and then refrigerate for at least 2 hours or until well chilled.
4. Make cake layers: Position rack in center of oven and preheat to 350°F. Lightly butter bottom and sides of two 8" x 2" round cake pans. Line bottoms of pans with parchment paper and butter paper. Flour paper and sides of pans, tapping out excess.
5. Put flour, baking powder and salt in medium bowl and stir with wire whisk to blend. Set aside.
6. Put sugar and zests in bowl of food processor fitted with metal chopping blade. Scrape vanilla bean seeds from pod and add to bowl. Process until zests are finely minced and sugar is infused with flavors.
7. In bowl of electric mixer, fitted with paddle attachment, beat butter and infused sugar on medium speed for about 5 minutes, or until mixture is pale yellow and fluffy. Scrape down sides of bowl. Add eggs, one at a time, beating for 30 seconds after each addition and stopping to scrape down sides of bowl as necessary. Reduce speed to low and add half of flour mixture. Beat just until absorbed. Remove bowl from mixer stand and, using large rubber spatula, fold in half of buttermilk. Fold in remaining flour mixture in two additions, alternating with remaining buttermilk in one addition.
8. Divide batter evenly between prepared pans and smooth tops with small offset metal spatula. Bake for 30 to 35 minutes, or until toothpick or cake tester inserted into center of cakes come out clean.
9. Cool cake layers in their pans on wire rack for 15 minutes. Run thin-bladed knife around edges of layers to loosen them. Invert layers onto racks. Peel off paper and leave paper loosely set on bottom of layers. Re-invert layers so they are right side up. Cool completely before frosting.
10. Assemble cake: Combine orange marmalade and liqueur in small saucepan and cook over low heat, stirring frequently, until marmalade is melted. Remove pan from heat.
11. Remove custard filling from refrigerator and whisk until smooth.
12. Using long serrated knife, cut each cake layer in half horizontally. Dust off excess cake crumbs. Set aside one bottom layer for top of cake. Place other bottom layer, cut side up, on serving platter or 8" cardboard round. Using small offset metal spatula, spread one-third of jam in even layer over cake. Spread one-third of pastry cream over jam to within 1/2" of edge of cake. Top with second layer and repeat layering twice more, ending with reserved bottom layer, cut side down. Press down gently on top of cake.
13. Make frosting: Melt both chocolates with butter in top of double boiler over barely simmering water, stirring until completely melted and smooth.
14. Remove pan from heat and add sour cream and vanilla. Beat with wooden spoon until well-blended.
15. Using small offset metal spatula, spread very thin layer of frosting (about 3/4 cup) over entire cake, filling in any gaps between layers. This is called the crumb coat: its purpose is to provide a smooth and even base for frosting. Chill cake for about 15 minutes, or until frosting is firm.
16. Frost cake: Spread remaining frosting over top and sides of cake. Let cake stand at room temperature for at least 1 hour before serving, to allow frosting to set and flavors to meld. Otherwise, refrigerate and bring to room temperature before serving.
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