1/2 cup (1 stick) butter, softened
Green food coloring
1 teaspoon pure peppermint oil (or 2 teaspoons peppermint extract)
1 pound (about 4 cups) confectioners’ sugar
Line a few baking sheets with wax paper. Beat the softened butter with a wooden spoon, then mix in the peppermint oil and two or three drops of green or red food coloring. Don’t add any more. The mints should be pastel, not lurid.
Stir in the confectioners’ sugar, then pour the contents of the bowl onto a clean work surface and knead with your hands until a firm dough is formed. If you absolutely must, you can add up to 1 tablespoon of water, but do this a teaspoon at a time.
Continue to knead for several minutes until the dough is smooth. (If you get this dough too wet, there’s no going back. Better to knead for a moment or two and see if it comes together before resorting to the water.)
Divide the dough into quarters and roll each into a ball. Set 3 dough pieces aside, covered with a sheet of plastic wrap or a damp, well-wrung-out dish towel. Halve a dough ball and roll each half out, like Play-Doh, into 2 long ropes 1/2 inch in diameter
Use a sharp kitchen knife to cut them into 1/2-inch lengths, so each mint is a little squarish pillow, pinched at each end, and lay the cut mints on the prepared baking sheets. Continue until you've cut all the remaining dough.
Let the mints rest, uncovered, at room temperature for several hours until they are firm and dry to the touch, shaking the sheets now and then to roll them around so they dry on all sides.
Store them in airtight tins (or plastic zip-top bags with the air pressed out, if you don’t have tins) for up to 2 weeks at room temperature, or up to 3 months in the freezer.