Step 1

Preheat the oven to 350F. Butter and flour 2 (9-inch) cake pans and line the bottoms with parchment paper. Wrap the pans with cake strips.

Step 2

In a large bowl, whisk together flour, baking powder, and salt.

Step 3

In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat butter on medium speed until creamy. Add the sugar and beat until light and fluffy, about 3 minutes. Add the eggs, one at a time, beating until well combined before adding the next.

Step 4

In a large liquid measuring cup, whisk together 1 cup buttermilk and vanilla.

Step 5

With the mixer on low, add one-third of the flour mixture. Alternate with half of the buttermilk mixture, and continue alternating with the remaining flour and milk mixture.

Step 6

Reserve 1½ cups (330g) of batter in a large bowl. Divide the remaining batter among the two cake pans.

Step 7

Melt the chocolate in the microwave in 20 second intervals, stirring between each, until smooth. Or melt in a double boiler. Add to the reserved cake batter along with the remaining ¼ cup buttermilk, and stir until fully combined.

Step 8

Dollop the chocolate batter over the plain batter in the pans. Using the tip of a knife, swirl both batters together.

Step 9

Bake for 30 to 35 minutes or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 20 minutes. Remove and cool completely on a wire rack.

Step 10

In a large bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed for about 2 minutes, until nice and creamy.

Step 11

Sift in the ¼ cup of cocoa powder. Add the salt and mix on low just until combined. Scrape the bowl down.

Step 12

With the mixer on low, beat in the confectioner’s sugar a few tablespoons at a time.

Step 13

Then, beat in the cream and vanilla. Turn speed up to medium and beat until the frosting is well combined and fluffy about 1 minute.

Step 14

Trim the tops of the cakes to create level layers if needed.

Step 15

Place one cake layer on a cake stand or plate. Spread about ½ cup frosting over the top. Place the other cake layer on top. Spread the remaining frosting all over the top and sides of the cake. Make swoops with a spoon or offset spatula, or decorate as desired. Chill for at least 1 hour or until ready to serve.