Preheat your oven to 350°. Generously grease two 9-inch cake pans with butter and dust with flour, tapping out any excess flour.
In the bowl of your stand mixer add the homemade cake mix. Withe the mixer on low, pour in the water and oil, mixing until incorporated. Add one egg at a time, mixing after each egg. Scrape down the sides and bottom of the bowl before adding in the vanilla, mix on low until combined.
Divide the batter (2-1/2 cups per cake pan), tap to release any air bubbles and slide into your preheated oven. Bake for 25 to 30 minutes or until a toothpick comes out clean when inserted into the center of each cake.
Allow the cakes to cool in the pan for 10 minutes before running a knife along the edge of the cakes. Place your hand on the tops of the cakes before slowly flipping the pans upside down to remove the cakes. Peel back the parchment paper and carefully flip the cakes right side up, placing them on a wire rack to cool completely before frosting.
FOR THE FROSTING:
In the bowl of your stand mixer fitted with the whisk attachment, add the butter and mix on low until creamy (about 2 minutes).
In the meantime, sift together the powdered sugar and cocoa powder into a large bowl.
Combine the half & half, buttermilk and vanilla.
Add a few large spoonfuls of the cocoa mixture alternating with the liquids. Mix on low speed until just combined. Increase the speed to high and mix for 4 to 5 minutes or until the frosting is light and fluffy.
Place the first cake layer onto a cake stand and top with 1/3 of the frosting. Starting at the center, spread the frosting out to the edge, leaving a little extra frosting overhanging the edge of the cake. Place the second cake layer on top of the frosting and top it with the remaining frosting. Spread the frosting out from the center and down around the sides of the cake.
If you are unable to find half & half, substitute with 1 cup whole milk combined with 1 cup heavy cream.