Combine the cocoa powder with the hot water and whisk until smooth. Set aside to cool.
Combine half the buttermilk with the vegetable oil and set aside.
Add the egg and vanilla to the remaining buttermilk and whisk to combine. Set aside.
Into the bowl of your stand mixer with the paddle attachment attached, add in your flour, sugar, salt, baking powder, and baking soda and mix for 10 seconds to combine.
While mixing on low, add in your softened butter and mix until the mixture looks like coarse sand.
While mixing on low, add in your buttermilk/oil mixture and mix for 2 minutes to develop the cakes structure.
Scrape the bowl and then continue mixing on low while slowing drizzling in the buttermilk/egg mixture until cohesive.
Divide the cake batter in half and add the chocolate mixture to half the vanilla cake batter and stir to combine.
Coat two 6"x2" cake pans with cake goop or another preferred pan release. Fill one pan with the vanilla mixture and the other with the chocolate.
Bake your cakes at 350ºF for 25-30 minutes or until the cake bounces back when you touch the top.
Let your cakes cool in the pan until they are barely warm to the touch but not cold.
invert your cakes onto a cooling rack to cool the rest of the way. I put my cakes in the freezer for 30 minutes (on the cooling rack) so they can cool down further and are easier to handle while stacking the cake. Watch my video above to see how to stack and decorate.