Chocolate Fudge Doughnuts

Name: Chocolate Fudge Doughnuts
Prep time: 30 min
Cook time: 10 min
Total time: 40 min
Cuisine: American
Ingredients: 130 g (¾ cup + 1 tbsp) plain (all purpose) flour; 90 g (¼ cup + 3 tbsp) caster sugar; 25 g (3 tbsp + 2 tsp) cocoa powder; 0.5 tsp baking powder; 0.5 tsp bicarbonate of soda (baking soda); 0.5 tsp salt; 90 ml (¼ cup + 2 tbsp) milk; 1 large egg; 1 tsp vanilla extract; 30 g (2 tbsp) unsalted butter, melted, plus extra for brushing your doughnut pan; 90 ml (¼ cup + 2 tbsp) boiling water; 0.5 tsp espresso powder (optional); 100 g (¼ cup + 3 tbsp) icing (powdered) sugar; 4 tsp cocoa powder; 30 g (2 tbsp) butter, melted; 2-4 tbsp milk, as needed; (optional) melted white chocolate, to decorate; (optional) chocolate curls, to decorate