Preheat the oven to 200 C. Line 10 muffin moulds with liners (recommend to prevent any chocolate chips sticking to the pan but you can also grease the pan if you don't have liners).
Combine the milk and lemon juice in a small bowl. Stir and set aside.
Sift the flour, baking powder, baking soda and salt together in a large bowl. Set aside.
In a mixing bowl, beat the butter and sugar until pale and fluffy, then add the egg and vanilla. Beat to combine.
Add 1/4 of the flour mixture (to stabilise the batter) and fold it in, then stir in the soured milk. Add the remaining flour mixture and fold the batter gently until just combined. Do not over-mix it but you can run a hand mixer through it on low speed if there are lumps. The batter will be on the thicker side.
Stir in the chocolate chips then divide the batter equally among the muffin moulds, filling them about 3/4th of the way. Sprinkle each one with a few more chocolate chips on top if you like (highly recommended!).
Bake for 5 minutes, then keeping the muffins in the oven, lower the temperature to 175 C and bake for another 15 to 20 minutes until the tops are puffed, browned and a toothpick poked in the center comes out clean. This varying temperature trick gives the muffins a high initial rise for extra rounded and domed tops.
Let the muffins cool in the tin for 10 minutes, then place on a wire rack to cool completely. Store in an airtight tin for 2 to 3 days at room temperature, then transfer to the fridge for a week or the freezer for about a month. Happy baking!