Heat oven to 350° F. Whisk together the flour, baking powder, and salt in a medium bowl; set aside.
In a separate bowl, beat the butter and brown sugar with an electric mixer on medium-high until light and fluffy, 2 to 3 minutes. Beat in the egg and vanilla, scraping down the sides of the bowl as necessary.
Reduce mixer speed to low. Add half the flour mixture, then the sour cream, and then the remaining flour mixture, mixing well between additions. Fold in the peaches and blueberries. (Don’t worry if there seems to be too much fruit in proportion to batter. The batter will rise around it while baking.)
Transfer the batter to an 8-by-8-inch or other 2-quart baking dish and sprinkle with the almonds. Bake until a toothpick inserted in the center comes out clean (aim for the batter, not the fruit), 1 hour to 1 hour, 20 minutes. Let cool slightly. Dust with the confectioners’ sugar and serve warm.
Must do: add 1/2 teaspoon of cinnamon to flour mixture. This brings out great flavor in fruits.