Preheat the oven to 375°F (190°C). Lightly grease a donut pan with nonstick spray.

In a large bowl, whisk together the flour, cocoa powder, baking powder, baking soda, salt, sugar, and instant coffee granules (if using). Stir in the mini chocolate chips.

In a separate bowl, whisk the eggs, buttermilk, vanilla extract, vegetable oil, and cooled brewed coffee (if using liquid coffee).

Add the wet ingredients to the dry ingredients and stir until just combined. Avoid overmixing to keep the donuts light and fluffy.

Transfer the batter to a piping bag or zip-top bag with the corner snipped. Pipe the batter into the donut pan, filling each cavity about 3/4 full.

Bake for 10–12 minutes, or until a toothpick inserted into the center comes out clean. Let the donuts cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.

Heat the heavy cream and butter in a small saucepan until steaming but not boiling. Remove from heat, add the chocolate chips, and let sit for 1 minute. Stir until smooth and glossy. Stir in the vanilla extract.

Dip the tops of each cooled donut into the glaze, letting the excess drip off. Place the donuts on a wire rack to allow the glaze to set.