Cream butter and sugar together until light and fluffy. Stir in flour and cornstarch. At this point, if the dough is too crumbly, add more softened butter, 2 tbsp at a time until you have a dough that can come together into balls.
Stir in chocolate and pecans. Roll into 1-inch balls and place on an ungreased baking sheet (you can put them fairly close together, they won't spread much). Bake 20 minutes at 350 degrees F. They should be very lightly browned.