Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.

Melt the butter (microwave or stovetop) and let it cool until warm/room temperature, about 10 minutes.

In a small bowl, whisk together the all-purpose flour, baking soda, and salt.

In a large bowl, whisk the cooled melted butter with the packed brown sugar and white sugar until combined.

Add the whole egg, the egg yolk, and the vanilla extract to the butter–sugar mixture and stir until fully incorporated.

Gradually add the dry ingredients to the wet ingredients, mixing until a dough forms. The dough may look crumbly at first; continue mixing or use your hands briefly until it comes together.

Stir in the chopped white chocolate until evenly distributed.

Use a 1- to 2-tablespoon scoop (or measure 1 to 2 tablespoons) to portion the dough. Roll each portion into a ball. (Optional: press additional white chocolate into the center of a ball if you want extra chunks.)

Pour the nonpareil sprinkles onto a plate. Roll each dough ball through the sprinkles, pressing gently so the sprinkles adhere.

Place the cookie balls on the prepared baking sheet, leaving about 2 inches of space between each.

Bake for 8 to 12 minutes, until the edges are slightly brown and the centers are soft and puffy. Do not overbake.

Let the cookies cool on the baking sheet for about 5 minutes, then transfer them to a wire rack to cool completely before serving.