Preheat the oven to 350°F. Grease an 8-inch square pan with butter. Beat the butter and sugar until light and fluffy, about 6-7 minutes. Add eggs and vanilla, mixing just until combined.

Combine flour and baking powder. Toss peaches with a few tablespoons of dry ingredient mixture and cinnamon. Gently coat the peaches.

Add half of the dry ingredients to the butter mixture and stir. Mix in sour cream. Add remaining dry ingredients and mix until just combined. Stir in peaches. Pour batter into prepared pan.

For topping, mix flour and brown sugar in the same bowl used for peaches. Add melted butter and stir until crumbly, leaving larger pieces intact. Sprinkle topping over the batter.

Bake for 44-46 minutes until a toothpick inserted in the center comes out clean. Cool slightly before slicing and serving.