Cookie dough: In a bowl of a stand mixer, fitted with a paddle attachment, beat the butter and sugar, at medium speed, until creamy. Add in the egg and vanilla extract, mix again to combine.
Whisk the flour, salt, and baking powder together in a separate bowl.
Add the flour mixture to the wet ingredients little by little. Mix at low speed until well combined. Make sure you scrape the sides of the bowl so it is all mixed in.
Chill dough: Roll the dough onto a parchment sheet until the dough is 1/4” thick. Transfer to a pan and place the dough in the refrigerator for 30 minutes.
Bake: Preheat the oven to 350°F. Add some parchment paper onto your baking sheet and set aside.
Remove the sheet of dough from the fridge. Use cookie cutters to cut the cookie shapes and place them on a baking sheet.
Bake for 9-11 minutes, or until the edges are golden. Let them cool for at least an hour.
Marble Icing: Mix your powdered sugar, meringue powder, 3 tablespoons of water and vanilla with a standing mixer using a whisk attachment. Beat the icing ingredients together on high speed for about 2 minutes. Place a spoon or knife into the icing, and lift it up. It should drizzle down and smooth out in about 5 - 8 seconds. If it looks too thick, add a bit more water little by little because it could easily get too thin. If it's too thin, add a bit more powdered sugar. The icing should be about a flooding consistency (like honey, should be a little runny, but not watery.)
Separate the frosting into three bowls and using a toothpick, add a drop of coloring onto it and swirl it on the white icing. If you are using two colors, repeat the same with another color in that bowl. You will notice the first color in this case, I used green, and as you swirl you will notice different shades across the icing. Continue by adding your second color onto the toothpick, in this case red, and do the same. Swirl it around the icing. Keep in mind that the icing will harden quickly, so make sure to cover anything not being used right away.
Dip the whole top of the cookie in the royal icing mixture. Make sure all of the edges are covered. Lift the cookie carefully and let the extra icing drip off the cookie by holding it over the bowl. Place it on a cooling rack to set, but keep in mind some of the icing might drip a bit.
Continue to do it for the color you like in each of the bowls. In the other one, I only used blue coloring on the white icing, and the last one I only used red coloring. Repeat the process for the cookies, until you have finished marbling them all. Let the cookies sit until the frosting has set for about 2 hours. Then serve & enjoy!
Notes
These iced cookies will stay fresh for up to 5 days if stored in an airtight container at room temperature.Place UNFROSTED cookies in an airtight container or freezer bag and freeze for up to 3 months. Thaw at room temperature before serving, then add the royal icing.