• Line a baking sheet with parchment paper. Set aside.

Make the cookies

• Mix the flour, cornstarch, baking powder, and salt in a bowl. Set aside.
• In another bowl, cream the sugar, vanilla, and margarine/vegan butter with a mixer for about 5 minutes, until it's fluffy.
• Slowly add the flour mixture and beat on low speed until it all comes together, but don't overmix.
• Add the milk and then use your hands to gather everything together. Form a ball with your hands.
• Place the dough on parchment paper and shape into a rectangle.
• Place another sheet of parchment paper on top and roll out the dough to about ¼ inch thick (about 0.5 cm). (I like using rolling guides for consistent, uniform thickness.)
• Place the dough in the freezer for about 15 minutes (I slide the parchment paper on a pizza pan and then put it in the freezer).
• Meanwhile, preheat the oven to 350°F / 180°C.
• After 15 minutes, take the dough out of the freezer, remove the top parchment paper and cut out as many shapes as you can with your favorite cookie cutter. Gather the dough scraps, roll it out again, and cut out a few more cookies.
• Place the cutouts onto the prepared baking sheet, each about an inch (about 2.5 cm) apart and bake for 10-15 minutes.
• Let the cookies cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely. (The cookies will be soft when they come out of the oven and they'll slightly harden when they cool.)

Make the icing

• Combine all the ingredients in a bowl and whisk together.
• If it’s too runny, add some more powdered sugar. If it’s too thick, add a tiny amount of milk, half a teaspoon at a time, until you get the desired consistency (see the pictures in the post).
• Add a bit of gel food coloring (if using). I like to swirl it in with a toothpick.
• Mix everything well to blend the color throughout.
• Frost the cooled cookies (I like using the back of a spoon to spread it on).
• Add some nonpareils or sprinkles (if using) and then let the cookies dry.