In a microwave-safe mug, combine the diced peach, all-purpose flour, granulated sugar, baking powder, ground cinnamon, and a pinch of salt. Use a fork or small whisk to mix the dry ingredients thoroughly until well blended.
Add the milk, vegetable oil, and vanilla extract to the dry mixture in the mug. Stir these ingredients together until you have a smooth batter without any lumps.
If you'd like a burst of extra peach flavor and texture, gently fold in a few additional peach chunks at this stage, being careful not to overmix.
Evenly sprinkle the brown sugar on top of the batter; this will form a delightful caramelized crust as the mug cake bakes in the microwave.
Microwave the mug on high for 1 minute and 30 seconds. Be mindful that cooking times can vary based on the wattage of your microwave; you may need to adjust accordingly.
After the initial cooking time, carefully remove the mug from the microwave (it may be hot!). Check the center of the cake; if it appears wet or unset, return it to the microwave for an additional 10-15 seconds.
Let the mug cake cool for about a minute before indulging in its warm, comforting flavors.
Notes
Serve warm with ice cream or whipped cream for an extra treat.