Grease a cookie sheet. Stir together flour, baking powder and salt. In bowl with mixer on medium, beat margarine for 30 seconds. Add the sugar, and beat until fluffy. Add eggs and vanilla, and beat well. Add dry ingredients, mix well. Cover and chill for one hour.
Divide dough into quarters, and roll each out to a 9 1/2-inch circle. Invert a 9-inch pie plate over the dough, and trim dough around it so you have a dough circle. Slice the circle into 8 pie- shaped wedges with a sharp knife. Put wedges on cookie sheet. Bend over each tip to form that hat tassel.
Bake at 375 degrees for 8 to 10 minutes. Cool completely.
Meanwhile, beat egg whites on high until soft peaks form. Beat in powdered sugar and lemon juice until stiff. Pipe white frosting on cookies to form the pompom of the hat, the brim of the hat and Santa's beard. Tint a small amount of the frosting red, and form the mouth. Tint a small amount of the frosting green and pipe on for the eyes.