Sourdough Sugar Cookie Bars

Instructions

1. Preheat the oven to 350 degrees F (175 degrees C). Line a 9x9 inch (or 8x8-inch) baking pan with parchment paper.
2. Cream the butter and sugar together in a large bowl until light and fluffy (about 2-3 minutes). You can use a hand mixer or stand mixer for this step.
3. Add the egg, sourdough starter, and vanilla extract to the bowl. Mix until well combined and smooth.
4. In a separate bowl, whisk together the flour, baking powder, salt, and cornstarch.
5. Gradually mix the dry ingredients into the wet mixture, stirring just until combined. Be careful not to overmix.
6. Fold in the sprinkles using a spatula, making sure they're evenly distributed throughout the dough.
7. Spread the dough evenly into the prepared baking pan. Use a spatula or your hands to press it into the corners and smooth the top. Note: this is a wetter sugar cookie bar dough, this is normal. It helps achieve optimal texture.
8. Bake for 28-35 minutes, or until the edges are lightly golden and a toothpick inserted into the center comes out clean. Note: if using an 8x8 inch pan they may need a little longer to bake.
9. Let the bars cool completely in the pan before frosting.
10. To make the frosting, combine the powdered sugar, heavy cream, and vanilla extract in a bowl. Mix until smooth and creamy.
11. Add the red food dye to the frosting and mix until the desired color is achieved.
12. Spread the frosting over the cooled bars and sprinkle with additional rainbow sprinkles if desired.