Cream the butter and sugar until light and fluffy. Beat in the egg yolk and vanilla extract. Sift together the flour, cornstarch, and salt; stir into the butter mixture. Shape dough into 20 to 24 small balls, about 1/2 inch in diameter. Roll each ball in the oats and place about 2 inches apart on cookie sheets.
Step 3
Bake 15 to 20 minutes until golden brown. Let cool on wire rack. Store in an airtight tin.