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 medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
In a large bowl, beat the softened unsalted butter and granulated sugar together using an electric mixer on medium speed until light and fluffy, about 3 minutes.
Add the egg and vanilla extract to the butter mixture and beat until fully combined.
Gradually add the dry ingredients to the wet ingredients, alternating with the whole milk, beginning and ending with the dry ingredients. Mix until just combined; do not overmix.
Spread the cookie dough evenly into the prepared baking pan, smoothing the top with a spatula.
Sprinkle the red, white, and blue sanding sugars evenly over the top of the dough to create a festive patriotic pattern.
Bake in the preheated oven for 18 to 20 minutes, or until the edges are lightly golden and a toothpick inserted into the center comes out clean.
Remove from the oven and allow the bars to cool completely in the pan on a wire rack.
Once cooled, lift the bars out of the pan using the parchment paper overhang and cut into 16 squares.
Serve and enjoy your festive 4th of July Sugar Cookie Bars!