Using an electric mixer (stand or hand-held), cream together butter, sugar, vanilla and salt until smooth, about 3 min. Scrape down sides of bowl, add egg and mix well. Scrape down sides of bowl again, slowly add flour and mix until a soft dough forms. Dust hands with flour and, in bowl, knead dough by hand until smooth.
Refrigerate, about 20 min. Dust a piece of parchment paper with flour, place dough in centre, dust top of dough with flour and cover with another sheet of parchment paper. Roll dough to about a 1/4-in. (5 mm) thickness. Remove top sheet of parchment (use it to line your baking sheet).
Cut into 1 x 2-in. (2.5 x 5 cm) bars or use cookie cutters to cut into other fun shapes, then place on prepared cookie sheet, spaced about 1 in. (2.5 cm) apart.
Preheat oven to 350°F (180°C) and bake on middle rack until set, 10 to 12 min. Let cool completely before decorating.