In medium-size bowl, stir together flour and salt.
In second medium-size bowl, beat together sugar, butter, sour cream, egg yolk and vanilla until creamy and smooth, about 2 minutes. Stir in flour mixture until blended. Shape into disk; wrap in plastic wrap; refrigerate for several hours or overnight.
Heat oven to 350 degree F. Lightly coat 2 baking sheets with nonstick vegetable-oil cooking spray.
Roll out dough on lightly floured surface to 3/8-inch thickness. Cut into rounds with 3-inch scalloped cookie cutter. Reroll scraps; cut out. Place on prepared baking sheets, spacing 1-1/2 inches apart.
Bake in 350 degree F oven 10 to 12 minutes or until lightly browned at edges. Remove cookies to wire rack; let cool completely. Makes 1-3/4 dozen cookies.
Using Royal Icing and stencils (see Note), paint holiday designs on cookies by holding stencil flat on top of cookie and, with clean 1/2-inch brush (used by artists), brushing colored icing through stencil onto cookie. Additional features may be painted freehand or piped on with a pastry bag.
Stencils may be made by tracing designs from books or tracing around cookie cutters onto parchment paper or waxed paper and then neatly cutting out the shapes.
In small bowl, beat together confectioners sugar, 2 to 3 tablespoons water and meringue powder until stiff and fluffy. Divide into 5 small bowls. Stir food coloring and a few drops water into 4 bowls; add just water to fifth bowl; icing should be thick and opaque but no longer hold a peak. Use a brush to paint the cookies.