Preheat the oven to 350ºF.
Grease a donut pan and set aside.
In a large bowl, cream together the melted butter, sugar, and eggs. Add in the buttermilk and vanilla extract, stirring until well combined.
Slowly mix in the cocoa powder, baking soda, and salt until well combined.
Mix in the flour, stirring until just combined.
Spoon the batter into the donut pan, filling them about 3/4 of the way full.
Bake for 8-10 minutes.
Remove from the oven and allow to cool for 5 minutes before transferring to a wire rack to continue cooling.
To make the glaze, combine the icing sugar, cocoa powder, vanilla extract, and milk into a medium sized bowl. Start with 4 tbsp of milk and add more as needed if the glaze is too thick.
Dip the tops of the donuts into the glaze and place them back onto the wire rack. Add sprinkles or other desired toppings and let set for 30-60 minutes before serving.