Preheat your oven to 350°F and line two baking sheets with parchment paper.
In a medium bowl, whisk together the flour, cornstarch, baking powder, salt, and baking soda until fully combined.
In a large mixing bowl, whisk the melted butter, granulated sugar, and brown sugar together until smooth and glossy.
Add the egg, egg yolk, and vanilla extract to the butter mixture and whisk until fully incorporated and creamy.
Fold the dry ingredients into the wet ingredients using a rubber spatula until just combined. Do not overmix.
Gently fold in the jimmies sprinkles until evenly distributed throughout the dough.
Scoop the cookie dough into large balls using a cookie scoop or about 3 tablespoons of dough each.
Roll each dough ball in additional jimmies sprinkles to fully coat the outside.
Place the cookies on the prepared baking sheets, leaving enough space between each cookie for spreading.
Bake for 10 minutes or until the edges are lightly set and the centers still look slightly soft.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a cooling rack to cool completely.
Serve warm or store in an airtight container for up to 5 days.