Created by Engrossed on January 21, 2012
Step 1: In large bowl, beat butter and shortening on high for 30 seconds or until softened.
Step 2: Add sugar and vanilla and beat until combined.
Step 3: Beat in the egg yolks.
Step 4: In a small mixing bowl, whisk together, flour, baking soda, cream of tartar, and salt. Beat into wet mixture.
Step 5: Shape into 1-inch balls and place 2 inches apart on ungreased cookie sheets.
Step 6: Bake at 300 degrees for about 20 minutes or until tops are just slightly cracked and sides are set. Don't let edges brown or they won't be soft!
Step 7: Transfer to wire rack to cool. These are best hot out of the oven...melt in your mouth butter and sugar YUM! They will become more crispy/crumbly as they cool, but are still delicious. You can microwave them for a few seconds to bring back the softness. Place in an airtight container. Can store at room temperature for up to 3 days. May freeze up to 1 month. Thaw for 15 minutes before serving if frozen.