Before baking bar cookies, line the bottom of your pan with parchment paper and butter it. Extend the paper above the edges of the pan. Allow the cookies to cool completely in the pan. Don’t use aluminum foil, it sticks to bar cookies.
You have to wait and let bar cookies cool first. It’s hard to wait, but if you want perfectly sliced bar cookies, you have to wait.
After cooling, place the cookie pan (with the bar cookie in it) in the freezer for 20 minutes or longer to harden.
Remove pan from the freezer. Lift the sheet of cookies from the pan and move it to the cutting board by grasping the edges of the paper and pulling upward. Peel off the parchment paper and discard it.
Use a ruler to mark the cuts with a knife.
First trim the edges straight and square. Put the cut off bits into a heavy Ziploc freezer bag and freeze it. Keep adding to the bag as you do your Christmas baking. These will make a great base for no bake cookies later.
Lightly score the top of the bar where you want to cut it.
The bar is frozen so you can't slice it; you have to lean on the knife and grasp it with both hands. With one clean motion cut all the way through the base of the cookie with your chef's knife*. Cut uniform squares, rectangles, diamonds or triangles.
*Use serrated knife to cut soft bar cookies. But partially frozen or frozen cookies require a heavier knife and chef's knife works the best.