Preheat the oven to 350°F. Grease and line two 8–9 inch cake pans (or three 6" pans) with parchment paper. Brush the sides with butter or oil.
Whisk dry ingredients. In a large bowl, combine flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add wet ingredients. Stir in the oil, butter, and milk until just combined. The batter will start to come together.
Mix in eggs. Add the eggs one at a time, mixing until fully incorporated after each. Stir in the vanilla.
Bloom the cocoa. Slowly stream in the hot chocolate while mixing—this deepens the flavor and darkens the batter.
Divide and bake. Pour evenly into the prepared pans. Bake for 30–40 minutes depending on pan size. Cake is done when a toothpick comes out clean and the top springs back.
Cool and frost. Cool in pans for 15 minutes, then run a knife around the edges and invert onto wire racks. Once cool, frost as desired.