For the sugar cookies: In a stand mixer fitted with the paddle attachment, beat the butter and sugar together just until combined, about 5 minutes. Add the vanilla and salt and beat a moment more, just to bring together.
With the mixer on low speed, gradually add the flour. Beat just until it comes together. Form the dough into equal-sized balls (about 24). Cover in plastic and chill for at least 30 minutes.
Preheat the oven to 350 degrees F.
On a lightly floured work surface, use a lightly floured rolling pin to roll the dough balls out to a 1/4-inch thickness. With a lightly floured cookie cutter, cut the cookies out in desired shapes and transfer them to a parchment-lined cookie sheet. Bake until the cookies are very light golden brown around the edges, about 10 minutes. Let cool completely before decorating with royal icing.
For the royal icing: Combine the confectioner's sugar, meringue powder and vanilla in a large bowl. With an electric hand mixer running on low speed, very gradually beat in 1/2 cup water, checking the consistency once most of the water has been added. Beat until the icing holds a ribbon, adding more water as needed. If too loose, add more sugar.
If coloring the royal icing, add a few drops of food coloring to the finished icing and slowly mix until completely incorporated.