Preheat your oven to 350°F (175°C) and generously spray a 6-cavity donut pan with nonstick cooking spray, making sure to coat every crevice.
In a large mixing bowl, whisk together the flour, cocoa powder, sugar, baking powder, and salt until no cocoa clumps remain and the mixture is evenly colored.
In a separate medium bowl, whisk the egg until the yolk and white are fully combined, then add the milk, melted butter, and vanilla extract. Whisk until smooth and slightly frothy.
Pour the wet ingredients into the dry ingredients and gently fold with a rubber spatula just until combined, about 15-20 strokes. The batter should be thick but pourable, with a few small lumps remaining.
Transfer the batter to a piping bag or large zip-top bag with one corner snipped off. Pipe the batter into each donut cavity, filling them two-thirds full to leave room for rising.
Bake for 10-12 minutes, until the donuts spring back when lightly touched and a toothpick inserted into the thickest part comes out clean or with just a few moist crumbs.
Let the donuts cool in the pan for 5 minutes, then carefully turn them out onto a wire rack to cool completely, about 20 minutes. They must be completely cool before glazing.
For the glaze, combine the chopped chocolate, heavy cream, and corn syrup in a microwave-safe bowl. Microwave in 20-second intervals, stirring between each, until the chocolate is fully melted and the mixture is smooth and glossy.
Dip the top of each cooled donut into the warm glaze, letting the excess drip back into the bowl, then return to the wire rack. The glaze will set in about 15 minutes at room temperature.
Repeat the baking process with any remaining batter, re-spraying the pan between batches. Serve the donuts once the glaze has set to a beautiful sheen.
