Warm the milk in a small saucepan on low heat. It should be just lukewarm, so you can dip your finger in it comfortably.
Add the flour, cocoa powder, sugar, yeast, bicarbonate of soda, and a pinch of salt to a large mixing bowl and mix to combine. Make a well in the centre and slowly pour in the milk, whisking all the time until you get a smooth batter. It’s important not to over mix, so stop as soon as the batter is mixed. Scrape any batter from the sides of the bowl and cover with greased cling film. Leave in a warm place to prove (but not too hot or the batter will collapse) for 2 hours, until it has doubled in size and has lots of bubbles.
Add the butter and maple syrup to a large mixing bowl and whip until well combined. Return to the fridge until serving.
Preheat the oven to 170°C/150°C fan/gas mark 4. Grease two crumpet rings with some of the sunflower oil, and place in a large non-stick frying pan on low heat. Add as many greased rings to the pan as you can fit and let warm up. Carefully spoon the batter into each ring to just cover the bottom. Sprinkle six chocolate chips over each crumpet and finish with another spoonful of batter until the crumpet ring is just under half full.
Cook the crumpets on a low heat for 10-12 minutes, and if the bottoms are burning, turn the heat down. Remove the rings with a tea towel and place the crumpets on a cooling rack. Transfer the cooling rack to the oven and bake for 4 minutes. Re-grease the rings and repeat the process until you’ve used up all the batter.
Serve the crumpets warm, topped with maple butter.