Double Chocolate Cake 
8 servings

CAKE 
2 cups sifted cake flour
1 teaspoon baking soda
1/2 teaspoon salt 
2 cups powdered sugar, divided
1 cup buttermilk, divided 
1/2 cup unsweetened cocoa powder
1 teaspoon vanilla extract 
1/2 cup (1 stick) unsalted butter, room temperature 
2 large eggs 

ICING 
3/4 cup water
1/2 cup sugar 1/2 cup (1 stick) unsalted butter, room temperature 
3/4 cup sour cream 
2 teaspoons vanilla extract 
2 cups sifted powdered sugar 
1 1/2 cups unsweetened cocoa powder
1/2 cup whole milk 

CAKE
Preheat oven to 350°F. Butter two 9-inch cake pans with 1 1/2-inch-high sides. Dust with powdered sugar. Line pan bottoms with parchment paper rounds. Sift flour, baking soda, and salt into medium bowl. Whisk 1 cup powdered sugar, 1/2 cup buttermilk, and cocoa powder in another medium bowl until smooth. Whisk remaining 1/2 cup buttermilk and vanilla in small bowl. Using electric mixer, beat butter in large bowl until creamy. Add remaining 1 cup powdered sugar; beat until smooth. Add eggs 1 at a time, beating to blend after each addition. Beat in cocoa mixture. Add flour mixture in 3 additions alternately with vanilla-buttermilk mixture in 2 additions, beating to blend after each addition. 

Divide batter between prepared pans. Bake until tester inserted into centers comes out clean, about 25 minutes. Cool 10 minutes in pans on rack. Invert cakes onto rack and cool completely. 

DO AHEAD
Can be made 1 day ahead. Cover and store at room temperature. 

ICING
Bring 3/4 cup water and sugar to boil in small saucepan, stirring until sugar dissolves. Boil until syrup is reduced to 3/4 cup, about 2 minutes. Cool. Using electric mixer, beat butter in medium bowl until smooth. Beat in sour cream and vanilla. Gradually beat in powdered sugar and cocoa powder, then 1/2 cup milk. 

Cut cakes horizontally in half. Place 1 cake layer on plate. Brush with 3 tablespoons syrup. Spread 1/2 cup icing over. Place second cake layer atop icing. Repeat sequence 2 more times. Brush top layer with remaining 3 tablespoons syrup. Cover top and sides of cake with remaining icing. 

DO AHEAD
Can be made 2 days ahead. Cover with cake dome and chill. Let stand 1 hour at room temperature before serving. 

Cut into slices and serve.