Preheat the oven to 350F. Prepare a 9×13 inch baking pan by lining the bottom with parchment paper and greasing the inner edges with butter
In a food processor, add shortbread cookies, popcorn, and salt. Chop until coarse crumbs. Add in butter and chopped together until it resembles a coarse almost paste texture.
Press mixture evenly and firmly onto the bottom of your pan (make sure the crust is packed tightly and evenly so you won’t have to deal with a possible seeping situation later).