Special equipment: a 12-ounce microwave-safe ceramic mug and a microwave oven
Put the flour, sugar, cocoa powder, baking powder and salt if using in a 12-ounce microwave-safe ceramic mug. Blend thoroughly with a fork. Add the milk, vegetable oil and vanilla if using and blend until smooth. Stir in the chocolate chips.
Microwave on high for 90 seconds. Do not overcook or the cake will be dry. Let cool for 2 to 3 minutes before eating.