Preheat oven to 200 C /Gas 6. Grease a 12 hole muffin tin (you may want to grease the top too!).
Sift the flour, cocoa, baking powder and salt into a large bowl. Stir in the chocolate, sugar and marshmallows.
Beat the eggs lightly in another large bowl. Beat in the milk and melted butter. Make a well in the dry ingredients and pour in the liquid ingredients. Use a spoon to mix gently until just combined and there are no pockets of dry flour visible. Don't overmix or beat! Spoon the mixture in to the holes of the muffin tin.
Bake for 20 minutes. The muffins should be well risen and firm to the touch.
When you take the muffins out of the oven leave them to cool in the tin for 5 minutes, then remove them from the holes by gently running a small knife around the holes and lifting them onto a wire rack.
The muffins can be frozen and warmed in a microwave before serving.