1. Preheat the oven to 350°F. Grease an 8-inch round cake pan and line the bottom with parchment paper.
2. In a large bowl, beat the butter, brown sugar, and granulated sugar for 2 to 3 minutes until light and fluffy.
3. Mix in the egg and vanilla until fully combined.
4. In a separate bowl, whisk together the flour, cornstarch, baking soda, baking powder, and salt.
5. Add the dry ingredients to the butter mixture and mix just until combined.
6. Fold in the white chocolate chips and patriotic sprinkles.
7. Press the dough evenly into the prepared pan.
8. Bake for 22 to 27 minutes, until the edges are lightly golden and the center is just set. It should still look slightly soft in the middle.
9. Let the cookie cake cool completely in the pan before removing.
10. Beat together the butter, powdered sugar, heavy cream, vanilla, and salt until smooth and fluffy.
11. Decorate: For a classic cookie cake, pipe a border of buttercream around the edge using a large star tip, then finish with extra patriotic sprinkles.

For the version shown, arrange the Pillsbury Flag Cookies evenly around the outside edge of the cookie cake, leaving a space between each cookie. Pipe a buttercream swirl in each gap using a 1M piping tip, then top the frosting with extra red, white, and blue jimmies.