Heat the oven to 180C/fan 160C/gas 4 and line a muffin tray with 10 cases. Whisk together the sugar, oil, whole milk and vanilla extract in a bowl. Crack in the egg, and mix until combined.
Fold in the flour to make a smooth batter. Add the chocolate chips and stir until fully incorporated. Divide between the cases and bake for 15-18 minutes until risen and golden on top. Move to a wire rack to cool. Will keep in an airtight container for two-three days.