Preheat oven to 325 degrees F. Spray the donut pan with cooking spray. Set aside.
Whisk together the dark brown sugar, flour, cocoa powder, baking powder and salt in a large bowl. Set aside.
In another bowl whisk together the melted butter, buttermilk, egg and vanilla.
Gently fold the wet mixture into the dry mixture. Stir until just combined. Next, add in the red food coloring and stir until combined.
Spoon mixture into the sprayed donut pan. Bake for 12-14 minutes (but start checking the doneness earlier). You want a toothpick inserted in the middle of a donut to still have some crumbs on it. You don’t want it coming out completely clean. (The donuts will be too done if it comes out completely clean.)
When they are done baking, remove the donuts from the pan and immediately place on a cooling rack.
Beat the butter, cream cheese and vanilla using a stand or hand mixer. Beat on medium-high speed until the icing is creamy. Reduce the mixer speed to low and slowly add in the sifted powdered sugar. Once the powdered sugar is combined add in the milk. Once the milk is combined turn the mixer speed to high and beat for 2 minutes.
Once the donuts are cooled, spread the icing on them. Enjoy!