Preheat your oven to 350°F (175°C) and grease a 9-inch cake pan or cast iron skillet with butter.

In a mixing bowl, combine softened butter and granulated sugar. Beat until creamy and light, about 2-3 minutes.

Add one egg, milk, and vanilla extract to the butter and sugar mixture. Mix until fully blended, about 1-2 minutes.

Whisk together all-purpose flour, baking powder, and salt in a separate bowl. Gradually fold dry mixture into wet ingredients, being careful not to overmix.

Pour the batter into the prepared cake pan and smooth the top with a spatula.

Place chopped peaches on top of the batter, pressing them down gently.

Mix together additional granulated sugar, flour, and cold butter until the mixture resembles coarse crumbs. Sprinkle over the peaches.

Bake in the preheated oven for 40-45 minutes or until the top is golden brown and a toothpick inserted in the center comes out clean.

Once baked, let the cake cool in the pan for 10-15 minutes before serving warm or at room temperature.