Preheat oven to 300 degrees F. Cream butter, sugar and vanilla until light. Add flour and mix just until dough begins to hold together. Roll into 1 inch balls and place, spacing them 1 inch apart, on a non-stick or parchment paper-lined cookie sheets. Slightly flatten cookies and then set a pecan on top of each, gently pressing it in. Bake cookies 20-25 minutes, or until they begin to very lightly brown.
Note: If freezing these cookies after baking, be sure store them, carefully arranged so they maintain their shape, in an air-tight containers.
When to serve these cookies: These cookies make an excellent addition to a decadent tray of sweets. There also are nice to serve their own with a hot cup of tea or coffee or cold glass of milk. They make a nice holiday season gift when packaged up in a decorative tin tied with a bow.