Preheat your oven to 350°F (175°C) and grease a donut pan with non-stick spray or softened butter.

In a large bowl, combine the all-purpose flour, cocoa powder, baking powder, baking soda, kosher salt, and light brown sugar. Whisk until well blended.

In a separate jug, whisk together the buttermilk, egg, melted unsalted butter, and vanilla extract until smoothly combined.

Gently fold the wet ingredients into the dry mixture using a spatula until no flour pockets remain.

Using a piping bag or a spoon, fill each donut cavity three-quarters full with the batter. Bake for 11-13 minutes or until a toothpick comes out clean.

Remove the donut pan from the oven and let the doughnuts cool in the pan for about 5 minutes.

In a medium bowl, whisk together the confectioners' sugar, milk, vanilla extract, and a pinch of salt until smooth and lump-free.

Dip the top of each doughnut into the prepared glaze and allow any excess to drip off. Place back on the wire rack to set.