• Preheat the oven to 350°F (175°C). Line a 9x9 inch (or 8x8-inch) baking pan with parchment paper.
• 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.
• Add the egg, sourdough starter, and vanilla extract to the bowl. Mix until well combined and smooth.
• In a separate bowl, whisk together the flour, baking powder, salt, and cornstarch.
• Gradually mix the dry ingredients into the wet mixture, stirring just until combined. Be careful not to overmix.
• Fold in the sprinkles using a spatula, making sure they’re evenly distributed throughout the dough.
• 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.
• 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.
• Let the bars cool completely in the pan before frosting.

How to make the frosting

• In a medium bowl, beat the softened butter until smooth.
• Add the powdered sugar, 2 tablespoons of heavy cream, vanilla extract, 1 drop of red food dye, and a pinch of salt (if using). Beat until smooth and fluffy. Add the remaining tablespoon of cream if needed to reach your desired consistency.
• Spread the frosting over the cooled bars, then top with extra sprinkles if you’d like.
• Slice into squares and enjoy!
• Store the frosted bars in an airtight container at room temperature for up to 2 days, or refrigerate for up to 5 days for maximum freshness.