Cream the butter and sugar together then slowly add the eggs. Mix for a few minutes then add the flour, baking powder and nutmeg or vanilla essence. Then add the blueberries. Stir to combine. refrigerate for at least an hour, preferably overnight.
Spoon the mixture into muffin cases, filling to just below the brim. Stud each muffin on top with a few blueberries.
Bake in the oven at 200C /400F /gas mark 6 for 20 minutes or until golden brown on top.