Preheat your oven to 350°F (175°C) and grease the doughnut pan with unsalted butter.

In a mixing bowl, combine Dutch-processed cocoa and instant espresso; pour in boiling water and stir until smooth.

In another bowl, mix flour, baking powder, and kosher salt.

In a large bowl, whisk the egg, then add brown sugar, whole milk, vegetable oil, and vanilla extract; whisk until smooth.

Gradually add the cocoa mixture to the wet ingredients and mix until just combined.

Fold in the flour mixture and chopped bittersweet chocolate to create the batter.

Spoon the batter into the prepared doughnut pan, filling each well about two-thirds full.

Bake for 10-12 minutes or until a toothpick inserted comes out clean. Let cool briefly in the pan before transferring to a wire rack.

Melt room-temperature butter and bittersweet chocolate together; dip the tops of the cooled doughnuts into the glaze, letting excess drip off.