Preheat oven to 400 degrees.

In a medium bowl, whisk the egg and stream in the melted butter until thickened, about 30 more seconds. Add the brown sugar, granulated sugar, and vanilla.

In another bowl, mix the flour, baking soda, and salt. Alternate adding the dry ingredients into the butter mixture with the buttermilk in three additions, starting and ending with the dry mixture. The batter will be thick. Place in a piping bag and cut a dime-sized hole for the opening.

Spray two mini donut pans with cooking spray and pipe in the batter until 3/4ths full. Place in the oven for 7-8 minutes. Let cool in the pans for 5 minutes before turning out onto a cooling rack.

Mix the frosting ingredients together. Microwave in 30-second increments until it?s a drizzle consistency. Dip the donuts into the frosting and place sprinkles on immediately, mixing the frosting in between each donut. You may need to re-heat the frosting as needed every few minutes. Let sit out for a few hours to harden.