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Preheat oven to 350°F (177°C). Spray or lightly butter two 9-inch cake pans. Set aside.
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. Scrape down the sides and up the bottom of the bowl as needed. 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. Scrape down the sides and up the bottom of the bowl as needed. 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. Do not overmix. Use a whisk to rid any large lumps, if needed. The batter will be slightly thick.
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, as shown in the photo above. Pour remaining yellow batter on top of the chocolate. Using a knife, swirl the two batters together. Don’t worry if it’s not perfect.
Bake the layers for 22-27 minutes, or until a toothpick inserted into the center of one cake comes out clean. Mine usually take 24 minutes. During bake time, loosely cover the cakes with aluminum foil if you find the tops are browning too quickly. 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. Add 1-2 more Tablespoons confectioners’ sugar if frosting is too thin or 1-2 more Tablespoons of cream if frosting is too thick. Taste the frosting and add a pinch more salt if it’s too sweet.
If needed, level cakes to create a flat surface (I usually use a knife, be careful doing this!). Place 1 cake layer on a cake stand or large serving plate. Evenly spread about 3/4 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.