Buttermilk Marble Cake with Two-Toned Frosting

Adrashanti requested a marble cake for her 7th birthday. This recipe, adapted from Christopher Kimball's cookbook "Dessert Bible," did not disappoint. My efforts were rewarded with a tangy-sweet flavor, a tender crumb and a relatively level rise. The frosting, my go-to recipe, should be kept cold for best flavor and texture.

Cake:
2 c unbleached flour
1/2 c cornstarch
1 tsp baking powder
1/2 tsp baking soda
1/2 tsp salt
1 1/2 sticks unsalted butter, softened but still firm
1 1/2 c granulated sugar
5 large eggs, at room temperature
1 c buttermilk
1 tsp vanilla paste
1 tsp vanilla extract
2 oz bittersweet chocolate (I used 65%)
1 Tbsp cocoa powder
1/8 tsp baking soda

Set oven rack in middle position; preheat to 350. Butter two 9" cake pans and line the bottoms with parchment.

Sift together flour, baking powder, baking soda and salt; set aside.

Beat butter at medium speed in a stand mixer for 30 seconds. Continue beating, gradually adding sugar, until light and fluffy, 2 to 3 minutes. Add eggs one at a time, beating after each addition. Scrape down sides of bowl frequently. Beat in vanilla extract and paste.

Add half of flour mixture, beating to incorporate, then add half of buttermilk. When incorporated, repeat with remaining flour mixture and buttermilk. Scrape down sides of bowl and beat to fully blend.

Remove 1/3 of batter to a bowl. Stir in melted chocolate. Sift cocoa powder and baking soda together atop batter; stir in.

Divide white batter equally between prepared pans. Spread to cover bottoms. Dollop chocolate batter equally onto white batter in three large dollops per pan. With a cheese spreader or butter knife, swirl to marble, first in one direction and then at a 90-degree angle, marbling as minimally as possible to create the desired effect.

Place pans in oven; bake 25 minutes, or until top of cake springs back when lightly pressed in center. Remove to cooling rack; let rest 5 minutes. Run a butter knife around sides of pans; invert to remove, then set right sides up on a cooling rack. Let cool at least 1 1/2 hours.

Frosting:
2 c whipping cream
2 c good-quality white chocolate chips (containing cocoa butter and no hydrogenated oils)
3/4 c bittersweet chocolate chips
12 ozs mascarpone cheese

Place white chocolate chips and bittersweet chocolate chips in 2 separate heatproof bowls. Pour 1 1/2 cups cream atop white chocolate and 1/2 cup cream atop bittersweet chocolate. Place each atop a bowl of simmering water, whisking until chocolate is melted. Place bowls of ganache in freezer to chill.

When chilled, beat white chocolate ganache with chilled beaters until light and fluffy. Beat in 8 ozs mascarpone cheese. Take care not to overbeat. Repeat process with bittersweet chocolate ganache, beating until fluffy, then beat in remaining mascarpone.

Assembly:
Brush crumbs off cake layers. Trim dome off one layer if necessary. Place cake, cut or top side down, onto a cake pedestal. Top with bittersweet chocolate ganache, spreading to within 1/2" of edges. Place other cake layer atop ganache, top side up. Frost top and sides of cake with white chocolate ganache.