Preparation

Make the pudding

在一个中锅,搅拌糖,玉米淀粉,cocoa, and salt. Whisk in milk and heavy cream until smooth. Cook over medium heat, whisking constantly, until very thick and glossy, about 5 minutes. Remove from the heat. Add chocolate chips, butter, and vanilla, and continue whisking until smooth. Press through a fine-mesh sieve to remove any lumps. Transfer to a bowl, cover surface directly with plastic wrap, and set aside to let cool completely.

Make the lemon custard

在一个中锅,搅拌糖,玉米淀粉,and salt. Add the water and egg yolks, and whisk until completely smooth. Cook over medium heat, whisking constantly, until very thick, about 5 minutes. Remove from the heat, and whisk in the lemon juice, butter, and zest until smooth. Press through a fine-mesh sieve to remove any lumps, if needed. Transfer to a bowl, cover surface directly with plastic wrap, and set aside to let cool completely.

Make the cakes

Preheat the oven to 350°F. Butter and flour three 9-inch round cake pans, and line the bottom of each with a circle of parchment paper.

In a medium bowl, whisk together flour, baking soda, and salt. In the large bowl of a stand mixer, beat together sugar and butter until light and fluffy, 2 to 3 minutes. Add eggs one at a time, beating well after each addition. Scrape down the sides. Add vanilla, and beat until combined. Add buttermilk and flour mixture in three parts, ending with flour. Divide batter equally among prepared pans. Bake until cakes bounce back in the center when pressed, about 35 minutes.

Cool cakes in pans for 10 minutes, then turn out onto wire racks and set aside to let cool completely. Use a long knife to level the tops of each cake. Cut each cake in half horizontally to make 6 layers total. (Because the cake is tall, it’s critical the layers be flat and even for stacking.)

Make the buttercream frosting

In a large bowl, combine the butter, confectioners’ sugar, milk, vanilla, and salt. Beat with an electric mixer on medium speed until light and fluffy, about 2 minutes.

Assemble the cake

Place one layer on a 9-inch cardboard cake round. Spread half of the layer with about 1⁄2 cup of the chocolate pudding and the other half with about 1⁄2 cup of the lemon custard, spreading each almost to the edge, but not completely. Repeat with 4 more cake layers, pressing down very gently on each layer to level the cake so you end up with a cake that is one-half lemon and one-half chocolate. Top with the sixth and final cake layer.

Frost the top and sides of the assembled cake with the buttercream frosting, being sure to first mark which half is chocolate and which half is lemon. (You can do this by carving a shallow “C” and “L” in the frosting on the top of the cake, if you like.) The coating of frosting will be fairly thin and will be covered later with icing, so it needs to be smooth, but not visually perfect. (You will have a small amount of buttercream frosting left over to simply decorate the outside of the cake later, if you like.) Chill the cake for at least 2 hours or overnight.

Make the lemon fondant icing

In a medium pot, combine confectioners’ sugar, the water, lemon juice, and corn syrup. Heat over medium-low for 4 to 5 minutes, whisking constantly. Stir in vanilla and food coloring. Set aside to cool until thickened but still pourable, about 5 minutes.

Gently set the cake on a rack set over a rimmed baking sheet. Quickly whisk the lemon fondant icing one last time until smooth, and then evenly pour it over the lemon half of the cake to cover the top and sides completely, ending with a straight edge down the middle. (Adjust the speed of your pour to make a thick coating, and use an offset spatula to evenly spread the icing, where needed.) Briefly chill the cake on the rack, uncovered, while you make the chocolate ganache.

Make the chocolate ganache

Place chocolate in a medium heatproof bowl. In a small pot, bring heavy cream just to a boil over medium-high heat, then pour over the chocolate to cover completely. Let stand for 6 to 8 minutes without touching or stirring, then whisk until mixture is smooth and shiny.

Remove cake from the fridge and pour chocolate ganache over the chocolate half of the cake to coat completely, using an offset spatula to gently smooth out the coverage, and make a straight line across the center of the cake, dividing the chocolate half and lemon half. Decorate the top of the cake with the leftover buttercream frosting, if you like.

Chill the cake, uncovered, 2 hours or overnight before serving.