• Preheat the oven to 350°F (175°C) and line a 9x13 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 eggs, 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. If using sprinkles, coat them in the dry mixture to prevent sinking.
• Gradually mix the dry ingredients into the wet mixture, stirring just until combined. Be careful not to overmix. The mixture should be thicker than cake batter but softer than traditional sugar cookie dough.
• Spread the dough evenly into the prepared baking pan. Use a spatula or floured hands to press it into the corners and smooth the top. Note: this is a wetter sugar cookie cake dough, this is normal. It helps achieve optimal texture.

  It requires a little elbow grease to spread it, but I promise it is worth it!
• Bake for 28-32 minutes, or until the edges are lightly golden and a toothpick inserted into the center comes out clean.
• Let the cookie cake cool in the pan for 20-30 minutes before transferring to a wire rack to cool completely.

How to make the frosting

• In a medium bowl, beat the softened butter until smooth.
• Add the powdered sugar, 3 tablespoons of heavy cream, vanilla extract, & pinches of salt. Beat until smooth and fluffy. Add the remaining tablespoons of cream to reach your desired consistency. I like using the full 6 tablespoons for a whipped cream texture.
• Spread the frosting over the cooled sugar cookie cake and decorate with festive fruit or sprinkles if desired.
• Cut into squares to serve and enjoy!
• Storage: Store any leftover sugar cookie cake in an airtight container at room temperature for up to 2 days, or refrigerate for up to 5 days to keep it fresher longer. If frosted, it's best to keep it chilled, especially in warmer weather. Let it come to room temp before serving for the best texture.