In a microwave-safe mug, whisk together the all-purpose flour, brown sugar, baking powder, and salt until well combined.

Add the melted butter, milk, and vanilla extract into the dry ingredients in the mug. Gently stir until just combined.

In a separate small bowl, toss together the peach slices, brown sugar, lemon juice, and optional cinnamon. Spoon the peach mixture over the cake batter.

Place the mug in the microwave and cook on high for 1 minute. Check for doneness and cook in additional 15-second intervals if needed.

Carefully remove the mug from the microwave and let cool for about 1 minute before serving.