Marble Cake Recipe

The best marble cake Ive ever had. And heres how you can make it.
Ingredients
2 cups 236g cake flour spooned leveled
2 teaspoons baking powder
12 teaspoon salt
1 cup 16 Tbsp 226g unsalted butter softened to room temperature
34 cup 150g granulated sugar
12 cup 100g tightly packed light brown sugar
2 large eggs at room temperature
4 large egg yolks at room temperature
1 Tablespoon 15ml pure vanilla extract
23 cup 160ml buttermilk at room temperature
4 ounces 113g bittersweet or semisweet chocolate coarsely chopped

Milk Chocolate Frosting
1 and 14 cups 282g unsalted butter softened to room temperature
3 and 12 cups 420g confectioners sugar
34 cup 62g natural unsweetened or dutchprocess cocoa powder
14 teaspoon salt
1 teaspoon pure vanilla extract
14 cup 60ml heavy cream halfandhalf or whole milk
optional chocolate or rainbow sprinkles for decorating
Instructions
Preheat oven to 350F 177C. Grease two 9inch round cake pans line with parchment paper rounds then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans. If its helpful see this parchment paper rounds for cakes video post.
Make the cake 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 34 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 mediumhigh 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