Preheat the oven to 350°F (177°C) and line a sheet pan with parchment paper. Set aside.

Combine the cooled, melted butter and granulated sugar in a medium mixing bowl. Whisk until the mixture is thick and lightened in color, 1-2 minutes. Whisk in the egg yolk and artificial vanilla extract until smooth.

Add the flour, cornstarch, baking soda, and Kosher salt to the wet ingredients. Use a rubber spatula to fold the cookie dough until it's about 90% combined. Then, add the rainbow sprinkles and mix the dough just until there are no more streaks of flour. Don't over-mix! The dough will be a little soft.

Using a #20 cookie scoop (3 Tablespoons), scoop the cookie dough into 6 balls. Carefully roll the cookie dough balls in extra rainbow sprinkles for decoration. Place the cookie dough balls 2-3 inches apart on the prepared sheet pan.

Bake the cookies for 8-9 minutes, until the edges are set and the centers are still a bit under-done. Let the cookies cool on the sheet pan for about 5 minutes. Then, transfer them to a wire rack to cool completely and enjoy!