Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
In a large bowl, cream together the softened butter, granulated sugar, and light brown sugar using an electric mixer on medium speed until light and fluffy, about 3 minutes.
Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract until combined.
In a separate bowl, whisk together the all-purpose flour, baking powder, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Fold in the white chocolate chips evenly into the cookie dough.
Spread the cookie dough evenly into the prepared baking pan, smoothing the top with a spatula.
Bake for 22 to 25 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out with a few moist crumbs.
Remove the pan from the oven and immediately sprinkle the red, white, and blue sprinkles evenly over the warm cookie bars.
Arrange the fresh blueberries and sliced strawberries on top in a festive pattern to resemble the American flag colors.
Allow the cookie bars to cool completely in the pan on a wire rack, about 1 hour.
Once cooled, use the parchment paper overhang to lift the cookie bars out of the pan. Cut into 12 equal bars and serve.