Lightly coat the bottom of a microwave-safe mug or coffee cup with cooking spray. Combine the flour, granulated sugar, cocoa powder, baking powder, and salt directly in the mug, stirring until evenly mixed. Pour in the milk, canola oil, and vanilla extract, mixing thoroughly to achieve a smooth batter, ensuring the bottom of the mug is fully incorporated. Gently fold in the chocolate chips or scatter them over the top. Microwave the mug on high for 70 to 90 seconds, until the cake is set yet slightly glossy on the surface. Let the mug cake sit undisturbed in the microwave for one minute before enjoying.