You'll need a unicorn cookie cutter for this recipe. I use fondant on these cookies. If you dislike the taste of fondant, then I recommend replacing it with basic royal icing (un-tinted). If using the royal icing, place the black sugar pearl in the glaze while it is still wet. Let the icing harden completely before decorating.
In an electric mixer fitted with the paddle attachment, mix the butter and sugar together until just incorporated. Do not over-mix at this stage, or the cookies may spread while baking.
Add the egg and vanilla extract. Mix again on low speed, stopping to scrape down the sides of the bowl intermittently as needed.
In a medium bowl, whisk together the flour and salt. Add to the butter and egg mixture. Mix on low speed until a dough is formed and there are no longer any streaks of butter in the mixing bowl. The dough will often clump around the paddle attachment while being mixed. This is normal and a good sign that your dough is the right consistency. If your mixture does not come together and is crumbly, add ice cold water 1 tbsp. at a time until the dough clumps. Roll the dough flat between sheets of parchment paper and chill until ready for use, at least 30 minutes.
Preheat the oven to 350°F. Line two baking sheets with parchment paper.
Use a 4.5-inch unicorn cookie cutter to stamp shapes from the dough and transfer them to the prepared pans. Bake cookies for 12-15 minutes, or until the cookies are lightly brown on the edges. Transfer the cookies to a wire rack to cool completely.
Remove the fondant from the packaging and knead well with your hands for 3-5 minutes. If the fondant sticks to your hands at any time during the kneading, dust it with 1 tablespoon of powdered sugar. Lay a piece of parchment paper out onto a work surface and flatten the fondant slightly. Top with a second piece of parchment and roll using a rolling pin to 1/4-inch thickness. Use the unicorn cookie cutter to stamp shapes from the fondant. Re-roll scraps and cut more unicorn shapes.
Brush a little water on the backs of the fondant unicorn shapes and place them onto the cookies. Mix gold lustre dust and drops of clear vanilla extract in a small bowl until a medium-bodied paint is formed. Brush the paint over the horn, mane, tail and hooves; let dry about 10 minutes. Use the fine-tipped black marker to outline the mane, horn, tail and hooves of each unicorn. Move the side of the pen against the top edge of the fondant to completely outline the shape. Draw diagonal lines on the horn, a small triangle on the ear, and an ‘L’ shape nostril at the end of the muzzle. Load a dry brush with red powdered food color and blot on a piece of scrap paper. Gently and lightly brush the powder onto the cookies in a circular motion. Deepen the color at the cheeks, hind legs and forelegs; leave the body of the unicorns mostly white.
Keep cookies sealed airtight for longevity. Cookies keep for about 2 weeks.