Special equipment: a squeeze bottle for easy decorating
In a large bowl, use a whisk to combine the flour, salt and baking powder. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar together on medium speed until light and fluffy, 3 to 5 minutes. Add the eggs and vanilla extract and beat until completely combined.
Reduce the mixer speed to low and add the flour mixture slowly, about 1/2 cup at a time until finished, mixing until just combined. Form the dough into 2 disks and wrap in plastic. Refrigerate until slightly firm, about 1 hour, or up to overnight.
Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment paper, or spray with cooking spray.
Roll the dough out onto a floured work surface and use cookie cutters to cut out shapes, then transfer to the prepared baking sheets, leaving about a 2-inch space between each cookie.
Chill the cut-out cookies on the baking sheets for about 15 minutes before baking for a more precise shape. Bake until just starting to become crisp and golden around the edges, about 15 minutes. Remove the cookies to a rack and let cool completely.
Use food coloring to tint Royal Icing into different colors. To decorate, cover the cookies with the Royal Icing, then sprinkle on decorations while the icing is still wet. Let the icing set completely.
Add the meringue powder to 1/2 cup water in the bowl of a stand mixer fitted with the whisk attachment and whip for about 30 seconds. Slowly add the confectioner's sugar, and continue to beat until medium peaks form. If the icing is too stiff, add water a tablespoon at a time. Keep covered until ready to use. Transfer to a squeeze bottle for easier decorating.