Preheat oven to 350°F (177°C). Grease two 9-inch round cake pans, line with parchment paper rounds, then grease the parchment paper.

Sift the cake flour, baking powder, and salt together in a large bowl. Set aside.

With a handheld or stand mixer fitted with a paddle attachment, beat the butter on high speed in a large bowl until smooth and creamy, about 1 minute.

Add the granulated and brown sugars and beat on high speed for 3-4 minutes until creamed.

With the mixer running on low speed, add the eggs then the egg yolks one at a time, then add the vanilla extract. Beat on medium-high speed until combined.

With the mixer running on low speed, add the dry ingredients in 3 additions, alternating with the buttermilk and mixing each addition just until incorporated.

Remove 1 cup of yellow batter and pour into a medium bowl. Melt 4 ounces of chocolate in the microwave, stopping and stirring every 20 seconds until completely smooth.

Pour chocolate into the 1 cup of yellow batter you set aside and stir until combined.

Pour an even layer of the yellow batter into each cake pan. Spoon chocolate batter on top.

Pour remaining yellow batter on top of the chocolate. Using a knife, swirl the two batters together.

Bake the layers for 22-27 minutes, or until a toothpick inserted into the center of one cake comes out clean.

Remove cakes from the oven and allow to cool completely in the pans set on a wire rack.

With a handheld or stand mixer fitted with a whisk attachment, beat the butter on high speed in a large bowl until smooth and creamy, about 1 minute.

Add confectioners’ sugar, cocoa powder, salt, vanilla, and cream with the mixer running on low. Once added, increase to high speed and beat for 3 full minutes.

If needed, level cakes to create a flat surface. Place 1 cake layer on a cake stand, cake turntable, or large serving plate.

Evenly spread about ¾ cup frosting on top. Top with 2nd cake and spread the remaining frosting all over the sides and top.

Decorate with sprinkles if desired. Slice and serve.

Cover any leftover cake and store at room temperature for 2-3 days or in the refrigerator for up to 5 days.