Preheat your oven to 350°F (175°C) and prepare an 8x8 inch baking pan by lining it with parchment paper or spraying it with non-stick cooking spray.

In a large mixing bowl, combine the large egg, granulated sugar, and brown sugar. Whisk until pale and glossy, about 3-4 minutes.

Melt the unsalted butter in a microwave-safe bowl in 30-second intervals until fully melted, then stir in the vanilla extract.

Slowly pour the melted butter and vanilla mixture into the egg and sugar mixture, whisking until just combined.

Gently fold in the all-purpose flour, cornstarch, salt, and sprinkles with a spatula.

Pour the batter into the prepared pan, spreading it evenly.

Bake in the preheated oven for 23-25 minutes, until the edges are golden and a toothpick comes out with moist crumbs.

Let the blondies cool in the pan for about 15 minutes before transferring to a wire rack to cool completely.