Whisk the dry ingredients - 3 cups flour, salt, baking powder in a small bowl.

Using a mixer on high speed, beat butter and sugar until well combined, about 3 minutes.

Add the egg, egg yolk, and vanilla; beat just to combine. Reduce speed to low and gradually add dry ingredients; mix just enough to combine.

Form dough into two ¾"-thick disks; wrap in plastic.

Chill at least 2 hours.

Preheat to 325°. Let 1 disk of dough sit at room temperature until softened slightly, about 20 minutes.

Roll out on a lightly floured sheet of parchment paper until about ¼" thick, dusting with flour as needed (if dough gets soft or sticky, chill on parchment until firm).

Cut out shapes with floured cookie cutters; transfer to parchment-lined baking sheets.

Pop the baking sheets in the fridge for 10 minutes before baking - this will help keep their shape when baking.

Bake cookies, rotating baking sheets halfway through, until edges are golden, 12–16 minutes, depending on size. Repeat with scraps and remaining dough.

Transfer to wire racks and let cool completely.

In a bowl, whisk together the icing sugar with the meringue powder, with 4 tablespoons of the water.

Using a hand mixer, on high, mix until the royal icing is drizzling at the right consistency from the whisk. If it is too thin, add more icing sugar. If it is too stiff, then add more water, teaspoon at a time.

Take 1/3 cup of the royal icing mixture, add one drop of purple gel food color, then mix until combined. Place into a piping bag. Take another 1/3 cup of the icing, and add one drop of black gel food color and one drop of purple gel food color, and mix until combined. Place into a separate piping bag.

Place the plain white icing in a deep bowl, wide enough for the width of at least one cookie.

Drizzle both colored icing over the uncolored icing, in alternating lines. Use a toothpick or cookie scribe to drag lines through the colors to create a swirled pattern. This pattern will create that marbled look royal icing sugar cookies!

Dip one cookie, face down straight into the marbled icing. Make sure the entire surface of the cookie touches the icing.

Slowly, lift the cookie, straight up, and shake it, to allow the excess icing to drip off. Place the cookie, icing side up on a wire rack.

Use the toothpick to clean the sides of the cookie, if there is still too much icing.

Marble the icing again, and repeat with each cookie, until done. Mix the icing so the white icing is on the top before you drizzle your two colored icing.

Let dry, until the royal icing is dried. It will take at least 3 hours, but preferably overnight.