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.

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.