Line a large tray, or several smaller trays, with parchment, wax paper, or a silicone mat. Set aside.

In a large mixing bowl or the bowl of a stand mixer, whip butter on high speed until light and fluffy.

With a fine mesh strainer, sift the confectioner’s sugar to remove any lumps or clumps.

Gradually add the confectioner’s sugar to the butter, 1 cup at a time, mixing well after each addition.

Mix in the whole milk, salt, and peppermint oil. Continue mixing until a crumbly dough forms.

Turn dough out onto a silicone rolling mat, or a countertop dusted with confectioner’s sugar.

Use your hands to bring the mixture together until a soft dough forms.

Divide into portions, one for each color you plan to make.

Add 1-2 drops of gel food coloring. With gloved hands, knead to evenly distribute the color thoughout the dough.

Wrap balls in clingfilm/plastic wrap. (This will prevent the butter mint dough from drying out while you work.)

Beginning with 1 color, roll dough into a long rope, about 1/2-inch thick.

With a sharp knife, slice the rope into 1/2-inch pieces.

With sugared fingertips or a small stepped palette knife, transfer mints to prepared tray.

Repeat with remaining colors.

Allow butter mints to sit uncovered at room temperature for 8-10 hours, or until dry.*

Layer between sheets of parchment or wax paper. Store in an airtight container at room temperature for up to 3 weeks.