Preheat oven to 350 degrees. Lightly grease a standard size doughnut pan, set aside.
In a large mixing bowl, beat the butter, brown sugar, and vanilla until creamy. Add the melted chocolate and mix until smooth. Add the egg and egg yolk, mix just until combined.
In a small mixing bowl whisk together the cocoa powder, flour, baking soda and salt. In a seperate mixing bowl combine the sour cream, buttermilk and vinegar. Alternate adding the dry ingredients along with wet ingredients. Mix until a smooth batter forms, taking care not to overmix
Scoop batter into a ziploc bag, clip the corner and pipe into the prepared molds, filling 3/4 of the way. Place in the oven and bake for 15-18 minutes or until a toothpick comes out clean. Take care not to overbake or they will turn out dry. I like to remove them as soon as they are slightly firm to the touch.
Allow the doughnuts to cool for 3 minutes before removing from the molds and placing on a wire rack to cool completely.
Meanwhile, to make the glaze, whisk together the ingredients in a small bowl. Dip the tops of the doughnuts into the glaze and shake/swirl to remove excess glaze. Return the doughnuts to the rack to allow the glaze to set, about 15 minutes (you can add sprinkles at this time if you'd like). Doughnuts are best the day they are made.