Preheat oven to 350F. Lightly spray donut pans with nonstick spray and set aside.
In measuring cup, measure out ⅔ cups milk. Stir in vinegar or lemon juice and set aside. Allow to sit for at least 5 minutes while you prepare the dry mixture.
⅔ cup milk, 2 tsp vinegar or lemon juice
In medium-sized bowl, whisk together flour, sugar, cocoa powder, baking powder, and salt.
1 ½ cups all purpose flour, ¾ cups sugar, ½ cups cocoa powder, 1 tsp baking powder, ½ tsp salt
In separate large bowl, stir together your milk and vinegar/lemon juice mixture, egg, vanilla extract, and melted butter.
1 egg, 1 tsp vanilla extract, 6 Tbsp butter
Add the dry mixture to the wet and stir briefly. Add mini chocolate chips and continue to stir until mixture is just combined (do NOT over-mix or your donuts will be dense)
½ cup mini chocolate chips
Spoon or pipe (using a large ziplock bag with a hole snipped in a corner) into donut pan.
Bake on 350F for 10-11 minutes.
Allow to cool before glazing.
Glaze
In small saucepan over low heat, combine chocolate chips and heavy cream.
1 cup chocolate chips, ⅓ cup heavy cream
Stir constantly until chocolate is completely melted.
Remove from heat and allow to cool for 5 minutes.
Dip donuts in glaze (or spread on top of donuts with knife) and cover with sprinkles, if desired.