Can I make this cake the day before?

Absolutely. In fact, making it the day before is a great idea. The cake has time to chill and the flavors come together beautifully.

Can I use regular milk instead of buttermilk?

Yes. You can make a simple buttermilk substitute by adding a little lemon juice or vinegar to regular milk and letting it sit for about 5 minutes.

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Start by making the chocolate cake. Preheat your oven to 350 and prepare two round cake pans with cooking spray and parchment paper.
In a large bowl, whisk together the flour, cocoa powder, sugar, brown sugar, baking soda, baking powder, and salt.
In another bowl, whisk together the eggs, buttermilk, vegetable oil, and vanilla. Slowly add the wet ingredients to the dry ingredients and mix until combined.
Divide the batter between the prepared cake pans and bake until a toothpick inserted into the center comes out with a few moist crumbs.
Let the cakes cool in the pans for several minutes before turning them onto a cooling rack. Make sure they are completely cool before frosting.
While the cakes cool, make the fudge frosting. Beat the softened butter until creamy. Add the cocoa powder and powdered sugar gradually, mixing between additions. Add the vanilla and enough heavy cream or milk to create a smooth, spreadable frosting.
Place one cake layer on a cake plate. Spread a thick layer of fudge frosting over the top. Add the second cake layer and cover the entire cake with the remaining frosting.
Use an offset spatula to smooth the frosting around the cake. Refrigerate before slicing so the layers and frosting have time to set.