Using an electric mixer, cream together butter, sugar, vanilla and salt until smooth, about 3 mins. 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 mins. 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" thickness. Remove top sheet of parchment.
Cut into 1x2" bars or use cookie cutters to cut into other fun shapes, then place on prepared cookie sheet, spaced about 1" apart. (You can use the parchment paper for this).
Preheat oven to 350 degrees and bake on middle rack until set, 10-12 mins. Let cool completely before decorating.