Preheat oven to 325F.
Whisk together flour, cocoa, baking powder and salt.
In a large bowl, whisk together Guinness, egg, sugar, brown sugar and butter.
Stir dry ingredients into the wet until just combined.
Spray doughnut pan with nonstick spray.
Fill each cavity 2/3 full.
Bake for 12 minutes.
Combine all glaze ingredients and stir until blended.
Dip the top of the donuts in the glaze and place on wire rack to dry.