1. Preheat the oven to 350°F (175°C). Line a 9×9 inch (or 8×8-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 an icing tool to spread 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 8×8 inch pan you may need to bake the sugar cookie bars a little longer.
9. Let the bars cool completely in the pan before shaping & frosting.

How to make the frosting

1. In a medium bowl, beat the softened butter until smooth.
2. Add the powdered sugar, 2 tablespoons of heavy cream, vanilla extract, 1 drop of red food dye or natural 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.
3. Set frosting aside to prepare for shaping.

How to Shape the Sugar Cookie Bar Hearts

1. Once the cookie bars are fully baked, allow them to cool completely. This step is important, cutting while warm can cause the bars to crumble or lose their shape.
2. Use a heart-shaped cookie cutter to cut out the cookies. You should be able to get about 9 heart shapes from one batch, depending on the size of your cutter. Press straight down for clean edges, and gently lift each heart out of the pan.
3. After shaping, frost each heart generously and finish with festive sprinkles or decorative toppings.
4. 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.