Preheat oven to 350°F. Line sheet pans with parchment paper or silicone baking mats.
Whisk together flour, baking soda, and salt in a medium bowl.
Using an electric mixer or stand mixer, beat the butter in a large bowl until creamy. Add the sugar and beat until light and fluffy.
Add the egg, egg yolk, and vanilla and beat until combined. Add the flour mixture and beat on low until just combined. Set the empty flour mixture bowl aside for use later.
Scrape the dough out onto a clean counter and divide it in three equal portions. Put one portion back into the large bowl, add the peppermint extract and food colouring (a couple drops to start, adding enough to make the dough red), and blend on low speed until combined and dough is red.
Put another portion of the dough in the empty flour bowl and add the cocoa powder. Mix on low speed until combined.
Pinch off about 2 teaspoons (or 15 grams) of each dough and press them gently together so they stick but keep the colours distinct. Roll into a ball, then roll the ball in sugar or sprinkles.
Place no more than 6 cookies on each prepared pan and bake one pan at a time. Bake for 12-14 minutes, or until the edges of the cookies are set (slightly firm) but centres are still very soft and puffed up. Remove from oven and let cool on pans for 10 minutes, then transfer cookies to a wire rack to cool.