Preheat your oven to 350°F (175°C) and lightly grease your donut pan with non-stick cooking spray or a thin layer of butter.
In a large bowl, whisk together the flour, cocoa powder, baking soda, and salt until no large lumps remain.
Add the sugar, melted butter, egg, milk, and vanilla extract to the dry ingredients, stirring with a spatula until just combined. Do not overmix, as this can lead to a tough donut.
Transfer the batter into a piping bag and pipe it evenly into the prepared donut cavities, filling them about three-quarters of the way full.
Bake for 9 to 10 minutes, or until the tops spring back slightly when touched.
Remove from the oven and let them cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
While the donuts cool, place the chocolate chips and heavy cream in a small microwave-safe bowl. Microwave in 20-second intervals, stirring in between, until the mixture is smooth and glossy.
Dip the top of each cooled donut into the chocolate glaze and set back on the rack to allow the topping to set.