Preheat the oven to 375 F/ 190 C/ gas 5. Grease 10-12 muffin cups or use paper cases. Cream the butter until soft. Add both sugars and beat until light and fluffy. Beat in the eggs one at a time. Sift together the flour and baking powder into the butter mixture alternating with the milk. Divide half the mixture between the muffin cups Sprinkle several chocolate chips on top, then cover with a spoonful of the batter. To ensure even baking, half-fill any empty cups with water. Bake until lightly coloured, about 25 minutes. Let stand for about 5 minutes before turning out.