Preheat your oven to 300°F (150°C) and grease a 13x9 inch baking pan.

In a large bowl, whisk together flour, sugar, cocoa, baking powder, baking soda, and salt.

In a separate bowl, whisk eggs until light, add oil, coffee, and vanilla, mixing well.

Gradually add dry ingredients to wet, alternating with buttermilk, stirring gently.

Pour batter into the prepared pan, lifting and dropping to release air bubbles.

Bake for 1 hour, or until a toothpick comes out clean; cool in the pan on a wire rack.

Beat butter and peanut butter together until smooth before gradually adding confectioners' sugar.

If too thick, add milk for a spreadable consistency.

Frost the cooled cake generously with the peanut butter frosting.

Slice into squares and serve at room temperature.