Preheat the oven to 350°F (175°C). Thoroughly grease a 12-cavity donut pan with softened butter or baking spray, paying special attention to the center posts.

In a medium bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt until evenly combined and no cocoa lumps remain.

In a second bowl, whisk the eggs, buttermilk, melted butter, and vanilla until smooth. Pour the wet mixture into the dry ingredients and stir gently just until no dry streaks remain; do not overmix.

Transfer the batter to a piping bag or a large zip-top bag with one corner snipped. Pipe the batter into the prepared pan, filling each cavity about two-thirds full.

Bake for 10 to 12 minutes, until the tops spring back lightly when touched and a toothpick inserted into the thickest part comes out with a few moist crumbs.

Cool the donuts in the pan for 5 minutes, then gently turn them onto a wire rack. Let them cool completely before glazing.

For the glaze, place the chocolate chips in a heatproof bowl. Warm the cream until steaming but not boiling, pour it over the chocolate, and let it stand for 2 minutes.

Whisk in the corn syrup and butter until glossy and smooth. Dip the rounded side of each cooled donut into the glaze, let the excess drip away, and add sprinkles or other toppings while the glaze is still wet.