Step 1

Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.

Step 2

Stir butter, white sugar, and brown sugar together in a bowl; add vanilla extract and mix well. Stir eggs, 1 at a time, into butter mixture until smooth.

Step 3

Sift flour, cocoa powder, and salt together in a separate bowl. Stir flour mixture, alternating with buttermilk, into butter mixture until batter is smooth, adding more buttermilk if needed to thin out batter. Pour batter into the prepared baking dish.

Step 4

Bake in the preheated oven until a toothpick inserted in the center comes out clean, 40 to 45 minutes.