Peach Cobbler Muffins

Name: Peach Cobbler Muffins
Ingredients: Nonstick cooking spray, for greasing the muffin tins (optional); 2/3 cup chopped toasted pecans; 1/4 cup plus 2 tablespoons all-purpose flour; 1/4 cup packed brown sugar; 4 tablespoons unsalted butter, melted; Pinch of ground cinnamon; 2 cups all-purpose flour; 2 teaspoons baking powder; 1/2 teaspoon ground cinnamon; 1/4 teaspoon baking soda; 1/4 teaspoon ground nutmeg; 1/4 teaspoon kosher salt; 1 cup granulated sugar; 1/2 cup buttermilk; 1/2 cup vegetable oil; 1/2 cup sour cream; 1 large egg; 2 teaspoons pure vanilla extract; 1 1/2 cups chopped peeled fresh or thawed frozen peaches (about 9 ounces), plus 18 slices (1/4 inch thick)