Lightly butter 2 (9-inch) cake pans and set aside.
In a large bowl, combine the flour mix, baking powder, xanthan gum and salt. Set aside.
Using a handheld mixer, beat the butter on high speed in a large bowl until smooth, about 1 minute. Add the granulated and brown sugars and beat on medium speed for about 5 minutes until creamy. Scrape the sides as necessary. Lower the mixer to low speed, then add the eggs and yolks one at a time and beat until incorporated. Add the vanilla extract and beat on medium speed until combined.
Add the dry ingredients in 3 additions, alternating with the buttermilk mixing until well combined. Do not over mix.
Reserve 1 cup of the yellow batter into a medium bowl.
Melt the 4 ounces of chocolate in the microwave, stopping and stirring every 30 seconds until smooth.
Combine the melted chocolate and the reserved yellow batter and stir until combined.
Pour an even layer of the yellow batter unto the prepared cake pans then spoon the chocolate batter on top and swirl using a knife. Refer to the picture above.
Bake for 22 to 27 minutes or until a toothpick inserted at the center comes out clean.
Remove the cake from the oven and cool completely.
FROSTING:
In a large bowl, using a handheld mixer beat the the butter until smooth about 1 minute. Add the cocoa powder, confectioner's sugar, salt vanilla and cream and beat on low speed then increase the speed to medium-high and beat for about 3 minutes until smooth. If it comes out too thin, add 1 tablespoon of confectioner's sugar at a time and if it's too thick, add more cream as needed.
ASSEMBLING THE CAKE:
Place 1 cake on a cake stand or large cake platter. Evenly spread ¾ cup frosting on top. Top with the 2nd cake and spread the remaining frosting all over the top and sides. Decorate with sprinkles if desired. Yum!
Recipe by The Gluten Free at https://www.theglutenfree.com/gluten-free-marble-cake-recipe/