Preheat the oven to 350º Fahrenheit. Line four 8” cake pans with parchment paper and spray with baking spray.
In a medium size bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
In a separate, large mixing bowl, whisk together the sugar, vegetable oil, brown sugar and melted butter.
Add in the eggs and yolk and whisk to combine.
Next, add in the buttermilk and sour cream and stir to incorporate. Then stir in the vanilla.
Add in the flour mixture and cocoa and whisk just until incorporated. Lastly, stir in the hot water. At this point, the batter should be fairly runny.
Evenly divide the batter into the 4 prepared cake pans. Place into the preheated oven to bake for 26 minutes or until a toothpick inserted comes out clean or with moist crumbs. Remove from the oven and place onto a cooling rack to cool completely.
MAKE THE FROSTING:
While the cakes are cooling, make the buttercream. In a stand mixer bowl, with the mixer on medium speed, whip the butter for 2 minutes until it is almost white in color.
Add in the powdered sugar, vanilla and 3 tablespoons cream. Beat on low speed for 2-3 minutes, adding in the last tablespoon of milk if the buttercream needs to be thinner. Then beat on medium speed for 2-3 minutes until the frosting is fluffy and spreadable.
ASSEMBLE THE CAKE:
Place one layer of cake onto a cake stand or plate. Using an offset spatula or rubber spatula, spread frosting over the surface.
Place a second layer of cake on top, ensuring that one edge does not hang over. Frost with more frosting. Repeat with the last 2 layers.
Once all of the layers are stacked, frost the outer edges using the remaining buttercream.
Slice and serve. Store in an airtight container at room temperature.