In a mixing bowl, whisk to combine flour, salt, baking soda and cornstarch. Set aside.

In the bowl of a stand mixer (you can also use a hand-held electric mixer), cream butter with sugars until light and fluffy - about 4 - 5 minutes, scraping down the bowl halfway through.

Add egg and mix until incorporated. Scrape down bowl.

Add vanilla and peppermint extracts. Mix again to combine.

Carefully add dry ingredients to the bowl and mix just until you no longer see pockets of flour.

Finally, add 1/2 cup of jimmies to the dough. Pulse the mixer (turn on and off) a few times to distribute the sprinkles throughout the dough. Using a rubber spatula, fold dough a few times, making sure to scrape the bottom and sides of the bowl to incorporate everything.

Use small cookie scoop to portion 24 balls of cookie dough. Place on cookie sheet and cover tightly with plastic. Refrigerate for at least one hour, up to overnight.

Preheat oven to 350°F (325°F convection) / 180°C / Gas mark 4.

Remove dough from fridge and top each cookie with more jimmies or nonpareils, and place on lined baking sheet.

Bake for 12 - 14 minutes, just until edges are golden brown. Let cookies cool on baking sheet for 5 - 8 minutes before transferring to cookie rack to cool completely.