Preheat oven to 325 degrees. Line a buttered 8-inch square baking pan with foil or parchment paper, allowing 2 inches to hang over sides. Butter lining (excluding overhang); set pan aside.
Whisk together flour, baking powder, and salt in a medium bowl, and set aside.
Put butter and sugar in the bowl of an electric mixer fitted with the paddle attachment; cream on medium speed until pale and fluffy, about 3 minutes. Add eggs and vanilla; beat until combined. Add flour mixture, and beat on low speed, scraping down sides of bowl, until well incorporated. Mix 3/4 cup almonds into batter.
Pour batter into prepared pan; spread with a rubber spatula. Scatter berries and remaining 1/4 cup nuts over batter in pan. Bake until a cake tester inserted into blondies (avoid center and edges) comes out with a few crumbs but is not wet, 55 to 60 minutes. Dust with confectioners' sugar before cutting into squares.