Gather all of your ingredients and equipment before you get started. 

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

Measure all of the ingredients. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In a large mixing bowl, beat the softened butter with the granulated sugar and brown sugar until light and fluffy, about 3 minutes.

Add the egg, egg yolk, and vanilla extract and mix until well combined.

Gradually add the dry ingredients to the butter mixture, mixing just until no dry flour remains. Fold in the sprinkles and white chocolate chips, if using. Be careful not to overmix, as this can cause the sprinkles to bleed their colour into the dough.

Using a cookie scoop or tablespoon, portion the dough into balls about 2 tablespoons in size. Roll the tops of each cookie in additional sprinkles for an extra festive finish.

Arrange the cookies on the prepared baking sheets, leaving about 2 inches between each cookie.

Bake for 10 to 12 minutes, or until the edges are just beginning to turn golden and the centres still look slightly underbaked.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a cooling rack to cool completely.