In a mixer using the blade attachment, cream together the butter and sugar. Add the eggs, one at a time, and mix for 4 mins. Add the flour, baking powder and milk, and mix for a further 4 mins on slow, then mix in the chocolate.
Heat oven to 200C/180C fan/gas 6. Cut 16 squares of baking parchment big enough to act as cases for the holes of your muffin tins. Using a piping bag or a spoon, fill the muffin cases with the mixture. When all the cases are filled, share the cherries equally between the muffins, pushing them into the mixture as you go. Bake for 25 mins, then serve warm or cold.