Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, allowing some to overhang for easy removal later.

In a medium mixing bowl, add the large egg along with both the granulated and brown sugars. Using a hand mixer, whisk on medium-high speed for 3-4 minutes until the mixture becomes pale and glossy.

Melt the unsalted butter in a microwave-safe bowl until just melted, which should take about 30 seconds. Stir in the vanilla extract.

Gradually pour the melted butter mixture into the egg and sugar mix while whisking at low speed. Increase the speed to medium-high and mix for an additional 10 seconds.

Gently fold in the all-purpose flour, cornstarch, and salt using a spatula until just combined. Fold in the sprinkles until evenly distributed.

Carefully pour the batter into your prepared baking pan, using a spatula to spread it out evenly.

Place the pan in the preheated oven and bake for 23-25 minutes. Check for doneness with a toothpick; moist crumbs should cling to it.

Remove the pan from the oven and let it cool for 15 minutes. Lift the blondies out using the parchment paper and transfer them to a wire rack to cool completely before cutting.