Preheat your oven to 350°F (175°C) and line a cupcake tin with paper liners. In a medium bowl, whisk together 1 ½ cups of flour, baking powder, and salt. Set aside.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the flour mixture. Mix until just combined. Gently fold in the diced peaches.

In a small bowl, combine the brown sugar, ½ cup of flour, and cinnamon. Cut in the cold, cubed butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.

Fill each cupcake liner about two-thirds full with the batter. Sprinkle a generous amount of the streusel topping over each cupcake.

Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Allow the cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.