Preheat your oven to 350°F (175°C) and prepare an 8" x 8" baking pan by lining it with parchment paper and lightly spraying it with non-stick spray.

In a large mixing bowl, combine the dry white cake mix, melted unsalted butter, egg, and pure vanilla extract. Stir until just combined.

Gently fold in the white chocolate chips and red, white, and blue sprinkles into the batter without overmixing.

Press about two-thirds of the dough evenly into the bottom of the prepared baking pan.

Pour the sweetened condensed milk over the pressed dough layer, spreading it out evenly.

Flatten the remaining dough and place it over the condensed milk layer, covering it as much as possible.

Bake the bars in the oven for 29 to 31 minutes until the edges are lightly browned and the center is slightly soft.

Let the bars cool in the pan for at least 60 minutes, then lift them out using the parchment and slice into squares.