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

Cover countertop with a silicone rolling mat. Dust with confectioner’s sugar.

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 out onto prepared silicone mat.

Use your hands to bring the mixture together, forming a ball.

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 one color, roll dough into a long rope, about 1/2-inch thick. (If using a smaller mat, you can roll 2-3 shorter ropes.)

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

With 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 (I allowed mine to try overnight) or until dry.

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