1. Preheat your oven to 350 F (175 C). Lightly grease and flour an 8×8 inch baking pan, or line it with parchment paper, leaving an overhang on the sides to easily lift the bars out later.

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

3. Beat in the egg and vanilla extract until well combined. Then, add the sourdough starter discard and mix until just incorporated.

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

5. Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Do not overmix.

6. Gently fold in the 1/2 cup of rainbow sprinkles with a spatula until they are evenly distributed throughout the dough.

7. Press the dough evenly into the prepared 8×8 inch baking pan. Sprinkle a few extra rainbow sprinkles over the top. Bake for 20-25 minutes, or until the edges are lightly golden and a toothpick inserted into the center comes out with moist crumbs. The center should look set.

8. Let the bars cool completely in the pan on a wire rack before slicing. Once cooled, lift them out using the parchment paper overhang and slice into squares.