Preheat oven to 350°F. Line a 10x15x1-inch baking sheet with parchment paper, then lightly grease the parchment with nonstick cooking spray. Lining the pan helps the bars lift easily after cooling and keeps edges from overbrowning.

In a medium mixing bowl, whisk together flour, baking soda, baking powder, and salt. Set aside.

In a large mixing bowl, beat together softened butter, granulated sugar, and powdered sugar on medium-high speed for about 5 minutes, scraping the sides halfway through. The mixture should become pale and fluffy, which helps the bars bake soft instead of dense.

Beat in the egg and vanilla extract until fully combined. The dough will look creamy and thick at this stage.

Gradually add the flour mixture to the butter mixture and GENTLY beat just until combined. Do not overmix at this point. The batter may look slightly crumbly, which is normal. It will press together when added to the pan.

Transfer sugar cookie dough into the prepared baking sheet and press evenly into the pan using your hands or the back of a spoon. Make sure the surface is level for even baking. Bake for 15 minutes, or until the cookie bars are lightly puffed and the top begins to crack slightly. Avoid overbaking; they’ll continue to set as they cool.

Remove from the oven and allow to cool completely in the pan before frosting. This prevents the frosting from melting into the cookie layer.

Spread prepared frosting evenly over the cooled bars using an offset spatula. Top generously with Christmas sprinkles or nonpareils.

Lift the parchment from the pan and place the cookie slab on a cutting board. Cut into bars or triangles. For standard bars, cut into 6x3 rectangles. For smaller holiday triangles, cut each rectangle diagonally into halves or quarters. The edges can be trimmed for cleaner presentation, but they’re perfectly tasty as is.