Preheat oven to 350 degrees F.
In a small bowl, whisk together flour, cream of tartar, baking soda and salt.
In a mixing bowl, cream butter, sugar and vanilla. Using a spoon, stir flour mixture into butter mixture and stir until well mixed. Stir in pecans and cornflakes.
Using a teaspoon, scoop up dough and shape into balls of about 1/2 to 3/4 inch. Place on ungreased or parchment lined cookie sheets, 12 per sheet, spacing a few inches apart. Bake for 13-16 minutes. Let cool on cookie sheet for about 4 minutes (they'll be delicate at first!) then transfer to a wire rack to cool completely.
Note: This cookie has no eggs and is easy to halve if you only want a small batch.
Note #2: My next experiment will be making these with Post "Banana Nut Crunch" cereal in place of Cornflakes.