Preheat oven to 350°F and line baking sheets with parchment paper.

In a large bowl, cream together butter, brown sugar, and granulated sugar until smooth and fluffy, about 2 minutes.

Add egg, egg yolk, and vanilla. Mix until fully combined.

In a separate bowl, whisk together flour, baking soda, and salt.

Add dry ingredients to wet ingredients and mix until just combined.

Fold in sprinkles (and white chocolate chips if using).

Scoop dough into 2 tablespoon balls and place on baking sheet.

Chill for 20–30 minutes (optional but helps with thickness).

Bake for 9–11 minutes, until edges are set and centers look slightly underbaked.

Press extra sprinkles on top right after baking for that bright, bakery look.