In a medium bowl, whisk together flour, baking soda and salt. In the bowl of stand mixer fitted with a paddle attachment, cream butter and sugar together until light and fluffy. Beat in egg and vanilla extract, scraping down bowl as needed. Slowly mix in dry ingredients until just combined.
Divide dough in half. Place one piece of dough between two pieces of lightly floured parchment paper and roll out to 1/4-inch thick slab. Repeat with remaining dough. Transfer both slabs to a baking sheet and chill at least 1 hour or overnight.
Preheat oven to 350°F. Line a large baking sheet with parchment paper; set aside.
Use a sweater cookie cutter to cut dough, re-rolling scraps as needed. Arrange 2-inches apart on prepared baking sheet. Chill unbaked cookies until needed.
Bake for 10-12 minutes, or until lightly browned on edges. Let cool 3-5 minutes on the baking sheet, then carefully transfer to a cooling rack to cool completely. Continue baking cookies until all of the dough has been used.
Decorate with royal icing (recipe below) to resemble ugly Christmas sweaters.
In the bowl of a stand mixer fitted with a whisk attachment, slowly mix powdered sugar and meringue powder until combined. Add 6 tablespoons water and beat on high speed for about 5 minutes, until glossy and stiff.
Add more water, a Tablespoon at a time, until desired thickness is reached for outlining or flooding the cookies. When not using, keep bowls of royal icing covered with a damp paper towel.