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 soda, and salt. Set aside.
In a large bowl, beat the softened butter, granulated sugar, and light brown sugar with an electric mixer on medium speed until creamy and smooth, about 2 to 3 minutes.
Add the vanilla extract and eggs one at a time, beating well after each addition.
Gradually add the flour mixture to the butter mixture, mixing on low speed until just combined. Do not overmix.
Fold in the semi-sweet chocolate chips evenly.
Spread the cookie dough evenly into the prepared baking pan and smooth the top with a spatula.
Bake for 22 to 25 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs.
Remove the cookie bars from the oven and allow them to cool completely in the pan on a wire rack.
While the cookie bars are cooling, prepare the frosting. In a medium bowl, beat the softened cream cheese and butter together until smooth and creamy.
Add the powdered sugar gradually, beating on low speed until combined, then add the vanilla extract and milk. Beat on medium speed until light and fluffy.
Once the cookie bars are completely cooled, spread the cream cheese frosting evenly over the top.
Decorate the frosted cookie bars by sprinkling the red sanding sugar on one third of the surface, the white nonpareils in the middle third, and the blue sanding sugar on the remaining third to create a patriotic red, white, and blue design.
Use the parchment paper overhang to lift the cookie bars out of the pan. Cut into 16 equal bars.
Serve immediately or refrigerate until ready to serve.