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 flour, baking soda, and salt. Set aside.
In a large bowl, using an electric mixer, beat the softened butter, granulated sugar, and brown sugar until creamy and smooth, about 2-3 minutes.
Add the vanilla extract and eggs one at a time, beating well after each addition until fully incorporated.
Gradually add the dry flour mixture to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Fold in the semi-sweet chocolate chips evenly into the dough.
Spread the cookie dough evenly into the prepared baking pan, smoothing the top with a spatula.
Sprinkle the red sanding sugar evenly over one third of the top edge of the dough, the blue sanding sugar evenly over the opposite third, leaving the middle third plain to create a red, white, and blue effect.
Bake in the preheated oven for 23-25 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs.
Remove from the oven and allow the bars to cool completely in the pan on a wire rack.
While the bars are cooling, melt the chopped white chocolate in a microwave-safe bowl in 20-second increments, stirring between each, until smooth and fully melted.
Drizzle the melted white chocolate evenly over the cooled cookie bars using a spoon or piping bag to enhance the festive look.
Once the white chocolate drizzle is set, use the parchment paper overhang to lift the bars out of the pan and place on a cutting board.
Cut into 16 equal bars and serve.