Preheat the oven to 375 degrees. Butter 2 cookie sheets and set aside. Roast 3/4 cup of pecans in oven for 10 minutes or until they emit a good aroma. Remove from oven and cool to room temperature. When cooled, chop pecans with sugar in a food processor until finely ground. Add flour, butter, egg yolk and salt. If dough is too soft, add up to 1/4 cup more flour; otherwise work dough with your hands.
Gather dough into a 1 1/2 inch round sausage, wrap in waxed paper and rest in fridge for 2 hours or overnight. Cut into 1/4inch rounds and set on baking sheet. Press a pecan half in center. Don't place cookies too closely together or they will run into each other as they spread during baking. Bake for 15 to 17 minutes or until the cookies' edges are golden brown. When they are baked they will give off a wonderful aroma. Remove baking sheets from oven and let cookies cool on sheets for 3 minutes. Then, remove them to a cookie rack to cool them further. Be careful when you handle these as they are very fragile.