Preheat the oven to 180C/350F/Gas 4. Grease a doughnut pan, and set aside.
Whisk together the flour, baking powder, baking soda, cocoa powder, salt, nutmeg, and zest. Set aside.
Whisk together the orange juice, egg, sugar, yogurt, and melted butter until combined.
Pour the wet ingredients into the dry ingredients, and mix until just combined. Do not over-mix.
Spoon the batter into the prepared doughnut pan and smooth out.
Bake for 5 - 7 minutes or until a toothpick inserted comes out clean. Allow to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
For the Glaze
Place the chocolate chips, butter, and syrup in a heat-proof bowl, and microwave until completely melted and smooth, stirring every 20 seconds. Stir in the orange juice.
Dip each doughnut into the glaze, garnish with chocolate sprinkles, and leave to set on the wire rack.
Notes
Leftover doughnuts can be stored in an airtight container at room temperature or in the fridge for up to 2 days. They also freeze well for up to 2 months. Thaw overnight in the fridge before serving.