Select a microwave-safe mug that holds at least 12 ounces. Lightly grease the inside with cooking spray or a thin layer of oil to prevent sticking.
Add the flour, sugar, baking powder, and salt directly to the mug. Whisk thoroughly with a fork for 20 seconds to break up any lumps and ensure the leavening is evenly distributed.
Pour in the coconut milk, vegetable oil, and vanilla extract. Stir vigorously with the fork until the batter is completely smooth with no dry pockets, about 30 seconds of mixing.
Gently fold in the diced mango and shredded coconut using a spoon, distributing them evenly throughout the batter without overmixing.
Microwave on high power for 60 to 90 seconds, checking at the 60-second mark. The cake is done when the top looks set and a toothpick inserted in the center comes out with just a few moist crumbs.
Let the mug cake rest for 30 seconds to finish cooking and firm up slightly. Top with extra toasted coconut or fresh mango if desired, then enjoy warm directly from the mug with a spoon.
