Add the flour, caster sugar, cocoa powder, baking powder and salt to a microwave-safe mug. Mix well with a fork.
Pour in the oil and milk. Mix until you have a smooth cake batter with no pockets of dry flour.
Fold in the chocolate chips or chopped chocolate. Reserve a few pieces to sprinkle on top, if desired.
Microwave on high for 90 seconds. The cake should be risen and cooked through while remaining soft and moist.
Enjoy warm as it is, or dust with icing sugar. For a more indulgent dessert, top with ice cream, chocolate sauce and extra chocolate chips.
Notes
Cocoa powder: This recipe uses 2 tablespoons for a balanced chocolate flavour. You can increase it to 3 tablespoons for a more intense chocolate cake, but it will be considerably richer.
Caster sugar: Caster sugar works best because its fine crystals dissolve quickly during the short cooking time. Milk: Dairy milk, almond milk, oat milk or soy milk can be used.
Oil: Use a neutral oil such as sunflower, vegetable or canola oil. Coconut oil can also be used, although it may add a subtle coconut flavour.
Chocolate: Chocolate chips are optional, but they add pockets of melted chocolate and help keep the cake moist. Chopped milk or dark chocolate works just as well.
Mug size: Use a wide microwave-safe mug with a capacity of at least 250–300 ml to give the cake enough room to rise.
Microwave: Cooking time can vary depending on your microwave power. Start with 90 seconds and add 10–15 seconds only if needed. Avoid overcooking, as this can make the cake dry or rubbery.
Best served warm: This mug cake is best eaten immediately while warm and soft. The texture isn't quite as good after sitting for a couple of hours.