1. Melt 1/2 tablespoon of butter in a microwave-safe mug with 3-4 peach slices and 1 tablespoon of brown sugar for 30 seconds to 1 minute.
2. In a bowl, whisk together 1 tablespoon of melted butter, 1 tablespoon of brown sugar, the egg, and 1 teaspoon of vanilla extract.
3. Gradually add in 1/4 cup of all-purpose flour, 2 tablespoons of old-fashioned oats, 1/4 teaspoon of baking powder, and 1/4 teaspoon of kosher salt. Stir until just combined.
4. Spoon the batter over the peach mixture in the mug; do not stir.
5. Add remaining peach slices on top and sprinkle with 1 tablespoon of brown sugar and a pinch of cinnamon. Add 1/2 tablespoon of butter on top.
6. Microwave on full power for 1 minute 45 seconds to 2 minutes.
7. Let sit for a minute to cool down, then enjoy!