Preheat the oven to 180C/350F and place 12 large muffin cases into a muffin tin. You can also make smaller muffins, you'll probably be able to make 16-18 small muffins with this mixture.
Place the sugar, lemon zest and eggs into a large bowl and beat until well combined and smooth.
Gradually add the melted butter, stirring between each addition until you have a smooth mixture.
Add in the melted chocolate and mix until well combined.
Add in the flour, baking powder, bicarbonate of soda, salt and cocoa powder and begin to mix gently. As the mixture starts to come together, add in the milk and continue to mix until there are no lumps of flour or cocoa powder and your mixture is thick and smooth.
To make jumbo muffins: place 3tbsp of mixture into each liner. To make normal sized muffins: place 2tbsp of mixture into each case.
Place in the oven for 15-20 minutes, until risen and an inserted skewer into the centre comes out clean. Smaller muffins will take 12-15 minutes.
Leave to cool in the pan for 10 minutes, before transferring to a wire rack with kitchen paper beneath it, to cool completely.
Once cool, make the glaze. Place the sifted icing sugar into a medium sized bowl and add in 1tsp of lemon juice, mix and then add another 1tsp of lemon juice, mix and see the consistency. After 2tsp mine was a thick paste, so I added the final tsp of lemon juice in. You want a thick consistency that will drop in ribbons off of your spoon.
Either with a spoon or a Ziploc bag with the corner snipped off, drizzle the icing mixture over each muffin. Leave to set for 15 minutes and then devour!
Chocolate Lemon Muffins will keep in an airtight container, at room temperature, for 4 days.