In a mixing bowl, whisk together the flour, cocoa powder, baking powder, baking soda, salt, and caster sugar.
In a separate bowl, beat the eggs lightly, then add the milk, vanilla extract, and melted butter.
Pour the wet ingredients into the dry ingredients and mix until just combined. Do not overmix.
Cover the bowl with clingfilm and leave to rest for 10 minutes.
Heat the oil in a deep pan over medium heat until it reaches 170°C.
Using a small ice cream scoop or two tablespoons, drop spoonfuls of the mixture into the hot oil, frying 2-3 doughnuts at a time. Cook for 2-3 minutes on each side or until golden brown.
Remove the doughnuts from the oil using a slotted spoon and drain on a wire rack lined with paper towels.
In a heatproof bowl, melt the dark chocolate and double cream together in the microwave or over a pan of simmering water. Stir until smooth.
Dip each doughnut into the chocolate glaze and decorate with sprinkles.
Leave to set for 10 minutes before serving.