Preheat oven to 350°F (175°C). Grease and flour two 8-inch round cake pans, lining the bottoms with parchment paper.
In a large bowl, whisk together flour, sugar, cocoa, baking powder, baking soda, salt, and espresso powder.
Add buttermilk, vegetable oil, eggs, and vanilla. Mix on medium speed for 2 minutes.
Stir in boiling water by hand or on low speed. The batter will be very thin.
Pour evenly into prepared pans. Bake for 30-35 minutes until a toothpick comes out clean.
Cool in pans for 10 minutes, then transfer to a wire rack to cool completely before frosting.