There are a couple of ways to make the crumbs. You can put cookies in a plastic bag and pulverize them using a rolling pin (fun for kids!), or you can whiz them in a food processor. I used a 10-oz. box of cookies and got about 1 1/3 cups of cookie crumbs.

This would also be delicious with graham cracker crumbs.

If you prefer a juicier crisp, omit or halve the starch. The starch thickens the juices and makes the filling more on the jammy side.