Peach Brown Betty Muffins

Name: Peach Brown Betty Muffins
Ingredients: Topping:; 3/4 stick unsalted butter, melted; 3/4 cup packed brown sugar; 1/2 cup all-purpose flour; 1/4 cup rolled oats; 1 1/2 teaspoons ground cinnamon; 1/4 teaspoon kosher salt; Muffins:; 2 1/2 cups all-purpose flour; 1 teaspoon baking soda; 1/2 teaspoon kosher salt; 1 1/2 cups packed brown sugar; 2/3 cup vegetable oil; 1 large egg; 1 teaspoon pure vanilla extract; 1 cup buttermilk; 3 large firm peaches, peeled, cored, and diced