Preheat your oven to 180°C (350°F). Grease your mini doughnut pan very well with butter (or line a mini muffin tin with paper liners).
Melt the butter in a medium pot over a low heat. Once melted, remove from the heat. Stir in the the sugar, milk and egg until smooth. Add the flour and baking powder and stir until mostly smooth.
Spoon the batter into the greased doughnut pan, filling it about 1/2 way full.
Bake for 8-10 minutes - they will be well risen but won't have coloured very much.
Remove from the oven, loosen from the pan and flip out onto a wire rack to cool. Clean and dry the doughnut pan, grease it, re-fill with the remaining batter and bake again.
Dip one side of the cooked doughnuts into the rhubarb glaze and set on a wire rack. Adorn with sprinkles quickly, before the glaze sets.
Store in an airtight container for up to 3 days.
These also freeze well - just freeze on a small tray for an hour then tip the doughnuts into a resealable bag and keep in the freezer for up to 3 months, defrosting at room temperature before eating.