Make the cake; preheat oven to 350℉. Line the bottom of your 9x13-inch baking dish with parchment paper and grease the sides of the baking pan with butter.
Mix the cake mix with buttermilk, eggs, vegetable oil, and vanilla extract.
Pour the batter into the prepared baking pan and bake for 35 minutes or until the inserted toothpick comes out clean. Once baked, remove the cake from the oven and allow it to cool completely.
Prepare the frosting:
Make the pecan frosting; combine condensed milk, butter, and egg yolk in a saucepot. Cook over medium heat for 5-6 minutes, or until the mixture has thickened. Remove from the heat and stir in coconut, pecans, and vanilla extract. Set aside to cool down.
Make the buttercream:
Beat the butter; beat the butter with a handheld mixer until creamy.
Add dry ingredients; add powdered sugar, cocoa, heavy cream, and vanilla extract. Add salt to balance flavor.
Start beating on low speed and then increase speed until you have a smooth buttercream.
Attach the desired nozzle to the piping bag. Fill the piping bag with the prepared buttercream.
Assemble the cake:
Spread around ½ cup of the buttercream over the surface of the entire cake. Use the remaining buttercream and pipe it around the cake to create a decorative border.
Spoon the pecan frosting into the center and spread carefully, trying not to smear the buttercream on the sides.
Cool the cake for 1 hour in the fridge before slicing and serving.
Notes
I used the baking mix that is 15.25oz. If you find the one that is 18.25oz. Make sure to increase the amount of buttermilk by ¼ cup.