Preheat oven to 350°F/175°C. Grease a donut pan with butter.
Whisk together the flour, sugar, cocoa powder, nutmeg, baking powder, baking soda, and salt in a large bowl. Combine buttermilk, egg, melted butter and vanilla extract.
Pour buttermilk mixture into dry mixture and fold in until all incorporated. Make sure there are no flour pockets.
Pipe the batter into the cavities, filling each ¾ of the way. Bake for 11-13 minutes, until golden brown and springy to the touch. After 5 minutes take out each donut and let cool on a wire rack, before pouring the next batch in. I like the re-grease the pan before I refill it.
In a microwave safe bowl, add in all of your glaze ingredients (except for the sprinkles!). Heat in the microwave in 25 second increments until melted, stirring after each 25 seconds. Do this until smooth. Dip the cooled donuts into the glaze and sprinkle over your sprinkles.
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. Just click on 'Metric' above the ingredients. Butter: I use European style butter, like Kerry Gold, in my recipes. Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Buttermilk: No buttermilk on hand? You can substitute it for sour cream or full fat yogurt. Make your own buttermilk in a pinch by pouring your half cup of milk in a jug or bowl. Remove ½ tablespoon of milk and add ½ tablespoon of lemon juice or white vinegar. Stir together and let it sit for 5 minutes before using. Storage: Keep the donuts in an airtight container and keep in the fridge for up to 3 days. You can freeze the donuts for up to 3 months, let them defrost overnight in the fridge before eating.