1. First, prepare your sugar cookie dough. In your electric mixer bowl, cream together the butter and sugar, then add the egg, milk and vanilla extract. In a small bowl, whisk together the flour, salt and baking powder. With the mixer running on medium speed, add the dry ingredients to the wet in three portions. Once you have a soft dough, shape it into a disk, wrap it in plastic wrap and let it rest in the fridge for about 30 minutes.
2. Take your dough out, and roll it out on a floured surface to about an 1/8 inch thickness. Preheat your oven to 350 deg F and line your baking sheets with parchment paper. Cut out your shapes, and place them on your baking sheets. Bake for about 8 minutes, or until the edges begin to brown very slightly. Cool completely before decorating.
3. Once the cookies have cooled completely, prepare your royal icing. Whisk all the ingredients except for the food coloring together (make sure it’s completely smooth and there are no lumps). You may need to add more milk or corn syrup, depending on the consistency you want. Add your food coloring in to get your desired color and decorate your cookies!