In a medium bowl, combine flour, baking powder, and salt, whisking well.
In a large mixing bowl, combine butter and sugars. Beat at high speed with a mixer until light and fluffy.
Add eggs and vanilla extract, beating to combine. Add flour mixture to butter mixture, mixing until incorporated. Add white chocolate morsels, strawberries, and blueberries, gently folding to combine.
Spread dough into prepared baking pan. Sprinkle with walnuts.
Bake until edges are golden brown and a wooden pick inserted in the center comes out clean, approximately 25 minutes.
Let cool completely on a wire rack. Cut into 24 (2-inch) squares.
Recipe by TeaTime Magazine at https://teatimemagazine.com/red-white-blue-blondies-recipe/