Cream margarine, sugar, and vanilla extract if using, with an electric mixer for one minute on high speed. Scrape down the sides and beat for another half minute. The mixture should look pale and fluffy.
Add flour and salt and mix on low speed, until just combined.
Transfer 2/3 of the cookie dough onto a piece of plastic wrap. (If you’re using a scale, around 400g.) Spread it into a rectangular shape (8×10 inches if you're being exact).
To the remaining dough, add a few drops of food colouring (and flavouring extract of choice, if desired) and mix again until colour is even all the way through.
Add the coloured dough on top of the first rectangle of dough and spread evenly to completely cover the first layer (see Step-by-Step Process pics for reference).
Roll the dough into a tight log, then wrap with the plastic wrap and twist the ends to close. Chill in the freezer, with the twisted ends tucked under the log, until firm (could be 1–4 hours depending on your freezer and the type of butter used).
The log should be quite firm but not completely frozen. Unwrap and slice into 1/2-inch thick cookies. (If your cookie gets squished by the knife, it's too soft; put it back into the freezer for another 10 minutes. If it crumbles when sliced, it's too frozen; leave it at room temperature for 5 minutes, then try again.)
Place on a lined baking sheet. These cookies do spread, so keep one cookie-width of space between each one.
Bake at 350 °F for 15 minutes (slightly soft in the center) to 20 minutes (totally crispy and crunchy).
Let cool fully on baking sheet. They will firm up as they cool.