Preheat the oven to 325 degrees. Lightly grease a donut pan.

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

In a small bowl, stir together the buttermilk, egg, vanilla, and butter.

Add the wet ingredients to the dry and stir with a wooden spoon until fully incorporated.

Fill the donut pan and bake for 10 minutes or until cooked through.

Remove from the oven and cool in the pan for a few minutes before transferring to a wire rack.

In a small sauce pan, heat the butter and peanut butter until melted over low heat.

Remove from the heat and stir in the powdered sugar and milk.

Dunk the donuts in the glaze when cool enough to handle.