1. Preheat the oven to 170°C (325°F), Gas mark 3, and line the muffin tins with paper muffin cases to make the number you require.
2. First make the sponge. In a freestanding electric mixer with the paddle attachment or using a hand-held electric whisk, mix the butter, flour, sugar, cocoa powder, baking powder and salt together until they form a crumb-like consistency.
3. In a jug, mix together the milk and eggs by hand.
4. With the mixer or whisk on a slow speed, gradually pour half of the liquid into the crumb mixture and mix thoroughly until combined. Raise the speed to medium and mix until the batter is smooth and thick, with no lumps. Scrape down the sides of the bowl from time to time. Once any lumps are gone, turn the speed back down and gradually pour in the remaining liquid, continuing to mix until the batter is smooth and combined.
5. Spoon the batter into the prepared paper cases, filling them two-thirds full. Using a 50ml (1 ¾ fl oz) ice-cream scoop can make this process easier and will result in even cupcakes.
6. Bake for 20-25 minutes or until the sponge bounces back when lightly touched. Leave to cool slightly before removing from the tin and placing on a wire rack to cool completely before frosting.
7. Meanwhile, make the cinder toffee. Line a baking tray with baking parchment, making sure the paper lines the base and sides of the tray. In a large, high-sided saucepan, mix the golden syrup, caster sugar and water together and bring to the boil. Boil until it reaches the hard crack stage (when a drop of boiling syrup immersed in cold water cracks) — about 150°C (302°F) on a sugar thermometer. Do not stir while the mixture is boiling or the caramel will crystallise; just gently swirl the pan from time to time.
8. When the mixture reaches the right temperature, whisk in the bicarbonate of soda — the mixture will froth up quite vigorously. Wearing oven gloves, carefully pour the toffee immediately into the prepared tray. Leave to cool and set completely.
9. Using a freestanding electric mixer with the paddle attachment or a hand-held electric whisk, gradually mix the icing sugar, cocoa powder and butter together on a low speed until combined and there are no large lumps of butter. Gradually add the milk while mixing on a slow speed. Once incorporated, increase the speed and beat until light and fluffy.
10. Break up the cinder toffee into small, rough pieces using a rolling pin. Mix the cinder toffee pieces into the frosting by hand, reserving some for decoration.
11, Once the cupcakes are cool, spoon generous amounts of the frosting onto each cupcake, then gently smooth over with a palette knife, making a swirl at the top if you wish. Top each cupcake with the remaining crushed cinder toffee.