Preheat the oven to 350°F. Line a 9x9-inch baking dish with parchment paper.

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

In a large mixing bowl, cream the softened butter and sugar together until light and fluffy.

Add the egg, sourdough discard, and vanilla extract to the butter mixture. Mix until smooth and fully combined.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.

Press the dough evenly into the prepared baking dish.

Bake for 20–22 minutes, or until the edges are lightly golden and the center is set. Let the bars cool completely before frosting.

To make the frosting, beat the softened butter until smooth. Add the powdered sugar, vanilla, and 1 tablespoon of milk. Beat until fluffy, adding more milk if needed for desired consistency.

Tint the frosting with food coloring if using, then spread evenly over the cooled cookie bars. Add sprinkles if desired.

Slice into squares and enjoy!