In a large bowl whisk together the dry ingredients. In a separate bowl whisk together the wet ingredients, then gently pour them into the dry ones. Fold ingredients together until just combined.
Spoon mixture into prepared doughnut pan, and bake for 13 – 15 minutes, or until the doughnuts spring back when lightly pressed. Allow doughnuts to cool a few minutes in the pan before transferring to a cooling wrack to cool completely.
While your doughnuts are cooling, make your glaze!
In a small heatproof bowl combine chocolate and butter. Place the bowl in the microwave and heat on low - in 20 second increments - until butter is completely melted and chocolate can be whisked completely smooth.
Whisk in half of the confectioners sugar, then add the sour cream. Add the rest of the sugar and a splash or two of milk; just enough to make it thin enough to dip the donuts.
Dip each cooled doughnut into the warm chocolate topping, then into sprinkles, if using.
Let doughnuts rest for 10 minutes (in a cool place, if possible) after dipping (so the glaze can harden), then EAT!