In a bowl, with an electric mixer on medium speed, beat butter, brown sugar, and vanilla until smooth. Stir or beat in flour until well blended.
Divide dough into four equal portions. Flatten each with your hands into a 1-inch-thick disk. On a lightly floured board, with a floured rolling pin, roll dough, a portion at a time, to 1/8 inch thick. With floured cookie cutters, cut dough into desired shapes (if dough becomes too soft to handle, freeze briefly until firm).
With a wide spatula or your fingers, transfer cookies to buttered or cooking parchment-lined 12- by 15-inch baking sheets, spacing them about 1 inch apart. Gather scraps, pat into a ball, and repeat rolling and cutting.
Bake cookies in a 300 oven until slightly darker brown, about 15 minutes; if baking more than one pan at a time, switch pan positions halfway through baking.
With a wide spatula, transfer cookies to racks to cool. If hot cookies start to break, slide a thin spatula under them to release, let stand on sheets to firm up, about 5 minutes, then transfer to racks to cool completely.
Powdered sugar icing: Controlling consistency of icing is simple. If it's too thick, add a few drops of water; if it's too thin, gradually add more powdered sugar. To tint, add food coloring a few drops at a time.
For piping: In a bowl, stir 2 cups powdered sugar, 1/2 teaspoon vanilla, and 1 1/2 tablespoons water until smooth. Tint with food coloring as desired. Spoon into a pastry bag fitted with a 1/8-inch tip or a heavy 1-quart zip-lock plastic bag. Seal plastic bag and squeeze icing into a corner; cut a tiny opening at corner of bag to squeeze icing out.
Let cookies stand until icing is firm, about 5 minutes. Makes about 3/4 cup.
