Cream the butter and sugar until light and fluffy. Add the egg and vanilla and mix well.

In a separate bowl, whisk together the flour and salt. Gradually add the dry ingredients to the wet ingredients and mix just until a soft dough forms.

Divide the dough in half, flatten each portion into a disk, and wrap in plastic wrap. Chill for at least 1 hour.

Roll the chilled dough on a lightly floured surface to about ¼ inch thick. I use ⅜ inch if I know kids will be helping because it makes the cookies sturdier.

Cut out mitten shapes and carefully transfer them to a parchment-lined baking sheet. Lift each cookie gently to avoid stretching the thumb or cuff. If the dough gets soft or sticky, return it to the refrigerator briefly before continuing.

Preheat your oven to 350°F (175°C). Bake for 8–10 minutes, or until the edges are just beginning to turn lightly golden.

Start checking at 8 minutes. Larger or thicker cookies may need a little longer.

Let the cookies rest on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.

Check the consistency by lifting the whisk. For coating the cookies, the icing should drizzle back into the bowl and smooth into the surface within about 5–10 seconds. Add the remaining water a little at a time as needed.

Reserve a small portion of white icing for decorations. Divide the remaining icing between two bowls, tinting one red and the other green with gel food coloring (or use whatever fun colors you want!).

Keep the white icing slightly thicker if you want to pipe lines and dots that hold their shape. Cover all three colors when you aren’t using them so the icing doesn’t begin to dry out.

Transfer the icing to separate piping bags or icing bottles fitted with small round tips.

Outline each mitten with your chosen base color, then fill the center with more icing. Use a toothpick or scribe tool to guide the icing around the thumb and into the cuff, fill small gaps, and pop air bubbles.

Use the reserved white royal icing to decorate the red and green mittens. Try simple dots, stripes, zigzags, or a white cuff at the bottom of each cookie.

For smooth, flat designs, add the white icing while the base color is still wet. Work with one or two cookies at a time so you can finish the details before the surface begins to crust.

For raised decorations with more defined edges, let the red or green base set before adding the white details. Use light pressure when piping so you don’t dent the base.

Let the finished cookies dry uncovered for at least 6–8 hours or overnight before stacking or packaging. Thick icing or humid conditions may require additional drying time.