Preheat oven to 350 degrees F (180 degrees C).
Slice Peach (2) and place into a skillet. Sprinkle Granulated Sugar (1 tablespoon) and Ground Nutmeg (1/4 teaspoon) over peaches and cook on medium for 5 minutes.
In a large bowl, cream Unsalted Butter (1/2 cup) and Brown Sugar (1/2 cup) until fluffy using a handheld mixer. Add Egg (1) and Vanilla Extract (1 teaspoon) and mix until fully incorporated.
In a small bowl, whisk Unbleached All-Purpose Flour (1 3/4 cup), Baking Powder (1/2 teaspoon), and Salt (1/2 teaspoon). Add to the butter mixture and mix until all flour is incorporated.
Spread 3/4 of the dough into a 9×9 pan.
Arrange peach slices over top and dot remaining dough over the top of the peaches.
Bake for 30 minutes or until light browned. Let cool on a wire rack before slicing.