Sift flour, measure and resift 3 times with soda and salt.
Cream butter until soft and smooth; add sugar and beat until well blended.
Add egg and beat until smooth and fluffy.
Stir in vanilla.
Gradually blend in the flour mixture.
Shape dough into small rolls about 2 inches in diameter, roll up in waxed paper.
Chill in refrigerator for several hours until firm; then turn out onto cutting board, strip off paper and cut into uniform slices about 1 3/6" thick or less, using a sharp, thin-bladed knife.
Transfer cookies to greased baking sheet, and bake in a moderately hot oven (400° F.) for about 8 to 10 minutes or until nicely browned.
Cool on cake racks.
Makes about 3 dozen.