Preheat oven to 350°F. Prepare two 8 inch round cake pans by spraying the insides with cooking spray and cutting round pieces of parchment paper to line the bottoms of the pans.*(SEE NOTES) Set aside.
In a medium bowl, combine flour, cocoa powder, sugar, baking soda, baking powder, and salt and whisk together until most clumps are gone.
Add in the milk, oil, vanilla extract, and vinegar and mix until lumps are gone.
Evenly divide the batter between the prepared pans. Bake for 16-23 minutes, or until a toothpick inserted in the middle of the cake comes out clean.
Place the cake pans on a cooling rack and leave the cakes in the pans until they've cooled completely. When removing the cakes from the pans, run a knife around the edges and turn them upside down onto the cooling rack. While they cool, make the buttercream.