Pre-heat oven to 325 degrees and lightly spray a doughnut pan with cooking oil.
In a large bowl, whisk together flour, sugar, baking powder, and salt.
In a separate smaller bowl, whisk together eggs, melted butter, Guinness, and chocolate milk.
Stir the wet ingredients into the dry ingredients until just combined, taking care not to over-mix.
Fill each cavity in the doughnut pan about 3/4 of the way with batter. I like to put batter in a plastic bag, cut off the corner, and squeeze the batter into the doughnut pan (you could also use a pastry bag).
Bake doughnuts for 8-10 minutes until they're cooked through and light and springy to the touch. Let cool in the pan for a few minutes before removing to cooling racks to cool completely. Repeat with second batch of doughnuts (if your doughnut pan only holds six).
Once doughnuts are completely cooled, frost with Irish whiskey frosting and add sprinkles, if desired.