Preheat oven to 350°F. Prepare a mini muffin pan with liners, or with non-stick cooking spray.
Combine flour, cocoa powder, baking powder, baking soda, and salt in a medium bowl and set aside.
In a medium saucepan over medium heat add in butter, chocolate, and espresso powder. Melt together while stirring.
Remove melted chocolate from the stove and add in vanilla extract, brown sugar, and sour cream. Mix until combined then add in egg yolk and vigorously whisk.
Add wet ingredients to the dry and stir until fully combined.
Using a mini ice-cream scoop (holds about 1 tablespoon of batter), drop rounded scoops into muffin pan.
Bake for 8-10 minutes, or until donut muffins have risen and cracked along the top.
Glaze
Prepare glaze while mini donut muffins bake. Whisk together confectioners sugar, milk, and vanilla extract until smooth.
Dip tops of slightly cooled muffins into glaze and set on a cooling rack that has been placed over a foil-lined cookie sheet.
Add sprinkles or embellishment if you prefer.
Notes
HIGH ALTITUDE BAKING: Use ¼ teaspoon baking powder, ¼ cup plus 2 tablespoons of brown sugar, ¾ cup sour cream, and bake at 375°F.