Preheat oven to 350°F. Lightly spray a 9-inch round cake pan or springform pan with cooking spray. If you would like to remove the entire cake from the pan before serving, line the bottom of the pan with parchment paper.

In the bowl of a stand mixer fitted with the paddle attachment (or using a large bowl and hand mixer), beat the softened butter and 1 cup granulated sugar together until light and fluffy, about 5 minutes.

Add the eggs one at a time, mixing well after each addition. Beat in the vanilla extract until combined.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gradually add the flour mixture to the butter mixture, alternating with the sour cream. Begin and end with the flour mixture, mixing just until combined after each addition. Be careful not to overmix the batter.

Gently fold the blueberries and diced peaches into the batter using a rubber spatula until evenly distributed.

Spread the batter evenly into the prepared pan. Sprinkle the top of the batter with ⅓ cup coarse sugar to create a sweet, crisp topping as the cake bakes.

Bake for 35–40 minutes, or until the top is lightly golden and crackly. The center should still have a slight jiggle, and a toothpick inserted into the center should come out with a few moist crumbs.