Preheat the oven 350°F. Lightly spray two 9-inch cake pans with baking spray. Place parchment paper in the bottom of the pan, spray the top of the paper with baking spray and set aside.
In a medium-sized bowl, stir together the cake flour, baking powder, baking soda, and salt, set aside.
In the body of a stand mixer with the paddle attachment cream the butter with the granulated sugar and brown sugar for 3 minutes.
Add the eggs one at a time until fully mixed in. Stir in the vanilla.
Add half of the dry ingredients and stir it in until combined.
Next, add half of the buttermilk, stir until combined. Repeat with the remaining dry ingredients and buttermilk, scraping down the sides as needed.
In a medium-sized microwave safe bowl add the chopped chocolate and microwave in 15 second intervals, stirring in between each interval until the chocolate is melted.
Add 1 ½ cups of the cake batter to the chocolate and stir until combined with no streaks.
Divide the yellow cake batter among the cake pans. Add dollops of the chocolate batter on top, make sure to put the same amount in each pan.
Take a toothpick or butter knife and gently swirl the two colors together.
Then, take a spatula and smooth out the top, this will ensure the cakes bake evenly.
Bake for 22-27 minutes or until a toothpick inserted into the center comes out clean. Let cool completely in the pan.
While the cakes are cooling, make the frosting. In the body of a stand mixer with the paddle attachment or in a large bowl with an electric hand mixer add the butter and whip until smooth.
Add the cocoa powder and carefully mix it in until combined. Add the powdered sugar a little at a time, mixing the first batch in completely before adding more, the mixture will be thick.
Lastly add the salt, vanilla, and 6 tablespoons of the heavy cream. Mix until smooth, if it appears too thick add an additional 1 tablespoon heavy cream. Once mixed in, turn the speed up to medium-high and beat for 3 minutes until light and fluffy. Scrape down the sides as needed.
To assemble the cake, take the cakes out of the pan and discard the parchment paper.
Trim the tops of the cake if there is any doming so they are even and flat.
Place one of the cakes bottom side down onto your serving dish or cake plate, add 1 cup of the frosting and smooth it out to the sides.
Place the other cake, top side down, on top of the frosting.
Take about another cup of frosting and lightly frost the entire cake, making a crumb coat then place in the refrigerator for at least 2 hours or up to overnight to set.
Once set, take the remaining frosting and frost the outside of the cake. Add chocolate curls around the outside edge if desired, cut and serve.