Preheat the oven to 350°F. Grease an 8" square pan with butter.

Beat the butter and the sugar together until light and fluffy, about 6-7 minutes. Add the eggs and the vanilla, mixing just until combined.

Combine the flour and baking powder in a bowl. Place the peaches in a small bowl and sprinkle with a couple of tablespoons of the dry ingredients, plus the cinnamon. Stir gently to coat the peaches.

Add half of the dry ingredients to the mixing bowl and stir to combine. Add the sour cream, mix again, and then add the remaining dry ingredients. Mix just until combined. Stir in the peaches and pour the batter into the prepared pan.

Place the flour and sugar for the topping in the same small bowl that was holding the peaches and add the melted butter. Stir lightly with a fork to combine, leaving plenty of larger pieces. Sprinkle the sugary crumbs over the batter in the pan.

Bake for 44-46 minutes, until a toothpick comes out clean or with moist crumbs. Let cool before slicing.