01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, ensuring an overhang for easy removal.
02 - Using an electric mixer or whisk, beat softened butter and sugar in a large bowl until light and fluffy, approximately two to three minutes.
03 - Add the egg and vanilla extract, beating until fully combined.
04 - Mix in flour and salt on low speed until just blended.
05 - Gently fold in 1/2 cup rainbow sprinkles, distributing evenly throughout the dough.
06 - Press dough evenly into the prepared pan and smooth the surface. Bake for 22 to 25 minutes until edges are lightly golden and center is set.
07 - Remove from oven and allow to cool completely in the pan.
08 - Whisk powdered sugar, 1 tablespoon milk, and vanilla extract until smooth. Gradually add more milk to achieve a thick but pourable glaze.
09 - Spread or drizzle glaze over cooled cookie base. Immediately scatter 1/4 cup sprinkles on top before glaze sets.
10 - Once glaze sets (about twenty minutes), lift slab from the pan using parchment and slice into sticks, each approximately 1x4 inches.