Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. Lightly grease the parchment paper if desired.

In a large mixing bowl, using an electric mixer, cream together the softened unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes.

Beat in the sourdough discard, egg, and vanilla extract until well combined and smooth, scraping down the sides of the bowl as needed.

In a separate medium bowl, whisk together the all-purpose flour, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix; overmixing can lead to tough bars.

Press the cookie dough evenly into the prepared 9x13 inch baking pan. You might find it easiest to lightly flour your hands or use a piece of parchment paper to help press the dough smoothly across the bottom.

Bake for 20-25 minutes, or until the edges are lightly golden and the center is set. A toothpick inserted into the center should come out with moist crumbs, not wet batter. Avoid overbaking for a softer bar.

Remove the pan from the oven and let the cookie bars cool completely in the pan on a wire rack before glazing. This is crucial for the glaze to set properly.

Once the bars are completely cool, prepare the glaze. In a medium bowl, whisk together the powdered sugar, 3 tablespoons of milk, vanilla extract, and a pinch of salt until smooth. If the glaze is too thick, add the remaining tablespoon of milk (or a tiny bit more) until it reaches a pourable but not watery consistency.

Pour the glaze evenly over the cooled cookie bars and spread with a spatula. If desired, immediately sprinkle with your favorite sprinkles before the glaze sets.

Allow the glaze to set for at least 15-30 minutes before lifting the bars out of the pan using the parchment paper overhang. Cut into 24 squares (or your desired size) and serve.