Sift maida with baking powder and keep it aside.

Mix the butter and maida mixture together with your fingertips until crumbly. Do not overmix. Add sugar and mix lightly. Mix in the chocolate chips and vanilla essence, if using.

Divide the milk into 2 parts. Add baking soda to one part and vinegar to the other. Mix the two milks; the mixture will start foaming. Add it quickly to the cake mix and mix fast and well. Transfer the mixture to muffin cups.

Pre-heat Oxy Fryer for 5 minutes. Arrange the muffin cups in the fry basket and close it properly. Keep Oxy Fryer at 140°C for 20–25 minutes, until a knife or skewer inserted inside comes out clean. Let the muffins cool for 10 minutes before removing them from the tin. Transfer to a wire rack and cool completely.