How to make crumbly cookies?
You want to minimize gluten formation, like in making pastry. So, use a low-gluten (pastry) flour. That recipe uses cornstarch alongside flour, which makes sense. You also want to, as much as possible, minimize water and coat the starch granules with fat. So, you might try spinning the flour and fat in a food processor before mixing in the wet ingredients. Definitely use the brandy -- same idea as vodka in a pie crust which helps to prevent gluten formation!