- Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper. 
- In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt. Set aside. 
- In a large bowl,  cream together the softened butter, granulated sugar, and brown sugar until light and fluffy. 
- Beat in the egg and vanilla extract until fully combined. 
- Gradually add the dry ingredients to the wet mixture, stirring just until combined. 
- Fold in the diced peaches, oats, and pecans if using, being careful not to overmix. 
- Drop rounded tablespoons of dough onto the prepared baking sheet, spacing about 2 inches apart. 
- Bake for 12-15 minutes, or until the edges are golden and the centers are set. 
- Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.