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, mix just until combined.

Combine the flour and baking powder. Dust diced peaches with a tablespoon of the dry mixture and cinnamon. Gently coat the peaches.

Add half of the dry ingredients to the wet mixture and stir. Mix in sour cream, then add remaining dry ingredients. Stir just until combined. Fold in peaches. Pour batter into the prepared pan.

Combine flour and sugar for topping in the same bowl used for peaches. Add melted butter and stir lightly to form crumbs. Sprinkle over the batter.

Bake for 44-46 minutes until a toothpick inserted comes out clean. Cool before serving.