Preheat the oven to 350°F. Coat a donut pan in butter or nonstick cooking spray.
In a large mixing bowl, use a whisk to beat together the milk, heavy cream, eggs, vanilla, and melted butter until homogenous.
Add the flour, cocoa powder, sugar, and baking soda to the mixing bowl. Gently whisk the ingredients together until combined and no clumps of dry flour or cocoa powder remain.
Fill the pan with batter and bake your donuts for 16 minutes.
Once your donuts have finished cooking, melt the chocolate chips in the microwave for about 45 seconds, or until fully melted.
Dip the top of each donut in the chocolate and then transfer to a wire cooling rack. Serve your donuts immediately or wait for the chocolate to set.