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.
In medium-sized bowl, whisk together flour, sugar, cocoa powder, baking powder, and salt.
In separate large bowl, stir together your milk and vinegar/lemon juice mixture, egg, vanilla extract, and melted 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)
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.
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.