Preheat the oven to 200 C / Gas 6. Grease a 12-cup muffin tin or line with paper cases.
Whisk together the flour, the 100g caster sugar, baking powder, salt and chocolate chips in a large bowl.
Stir together the milk, oil and egg in a small bowl. Pour into the flour mixture, and stir just until all ingredients are moistened. Fill muffin cups 2/3 full. Sprinkle with the remaining 3 tablespoons caster sugar.
Bake in the preheated oven until a skewer inserted near the centre comes out clean, about 20 minutes.
If you plan to serve these as cupcakes, omit the sprinkling of sugar at the end and let them cool completely before icing.