from AVERIE COOKS: http://www.loveveggiesandyoga.com/2012/06/old-fashioned-butter-mints.html 
Makes about 200 bite-sized mints
1/4 cup butter, softened
1/4 teaspoon salt
3 1/4 cups confectioners’ sugar plus 1/4 cup+, if needed
1/3 cup sweetened condensed milk
1/2 teaspoon peppermint extract*
food coloring, optional
To the bowl of stand mixer fitted with the paddle attachment, combine butter and salt and beat for 1 minute on medium-high speed. Add 3 1/4 cups confectioners’ sugar, milk, peppermint, and beat on medium-low speed until a dough forms. If the dough seems wet, add additional confectioners’ sugar until dough combines (I use 3 1/2 cups sugar). The dough will be crumbly but will come together when pinched and squeezed into a ball.
Taste the batter. If you want a more intense mint flavor, add additional mint extract, to taste.* Be very careful how much mint you add; you cannot un-do this. I repeat, be very, very careful with how much you add. Even 1/8th teaspoon peppermint extract can change flavors, dramatically.
Remove dough from the mixer, separate it into 1 to 4 smaller balls, and add one ball back into the mixer. Add the food coloring of your choice to the ball by squirting the droplets on top of the dough (careful when you turn on the mixer), and paddle on low speed until coloring is well-blended. Coloring will not blend completely into each and every speck of dough if examined extremely closely, but overall, mix until color is uniform. (I separated approximately two-thirds of the dough and made it green using about 15 drops green food coloring and made one-third of the dough red-pink by using about 7 drops red food coloring).
Wash the mixing bowl and the paddle in between each color change and repeat until all the balls are colored. After the dough has been colored, either wrap it with plastic wrap and place it in an airtight container in the refrigerator to be rolled out later or roll it immediately.
Place a golf-ball sized amount of dough in your hands and roll dough into long thin cylinders about 1 centimeter wide. Place cylinders on countertop and with a pizza cutter (or knife – be careful of your counter), slice cylinders into bite-sized pieces, approximately 1 centimeter long. Store mints in an airtight container in the refrigerator where they will keep for many weeks.
*A few notes about mint extracts: They are much more intensely flavored and potent than vanilla extract; 1 teaspoon of mint extract has an extreme amount of potency compared with 1 teaspoon vanilla extract. You cannot un-do mint once added so be very, very careful to not over-do it and end up with a bottle of Listerine-tasting food. There are different kinds of mint extracts available and are labeled as “mint, “peppermint”, “spearmint” and more. For this recipe I used store-brand (Kroger/Ralph’s) “peppermint extract” sold in a small 1 ounce bottle. Select the version of “mint” you think sounds best as not all types are available in all areas.
Recipe variations and thoughts: I suspect this recipe would be nice with cinnamon extract, lemon or orange extract, or many other specialty-flavored extracts from butter to rum to coconut to coffee extract. I have not tried making the dough first into a ball and then adding the extract after the dough has combined, thereby making it easier to customize the flavors from one big batch of mints into 50 pieces of orange, 50 pieces of cinnamon and so forth. Unfortunately, I have a feeling that extract added after the dough has combined would not disperse well and some pieces would be insanely strongly flavored and others would hardly be flavored at all. Working in an even smaller batch size is an option, although a bit challenging because less than one-quarter cup butter begins to be challenging.