Combine the flour, cocoa powder, baking powder, baking soda, salt, and caster sugar in a mixing bowl. Whisk the dry ingredients together.
In another bowl, lightly beat the eggs, then add milk, vanilla extract, and melted butter to the mixture.
Pour the wet ingredients into the dry ingredients and stir until just combined. Avoid overmixing.
Cover the bowl with clingfilm and let the mixture rest for 10 minutes.
Heat 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 let them drain on a wire rack lined with paper towels.
Melt dark chocolate and double cream together in a heatproof bowl using either a microwave or a pan of simmering water. Stir until the mixture is smooth.
Dip each doughnut into the chocolate glaze and add sprinkles for decoration.
Leave the doughnuts to set for 10 minutes before serving.
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