Secret to super thick cookies?

I'm obsessed with figuring out the secret to these cookies by Big Fat Cookie in Chicago! They're large 6oz thick cookies. The tops are shiny/crunchy while the inside has a fluffy (not cakey) crumb. When cooled they appear to be relatively cooked rather than super raw. The ingredients are: wheat flour, sugar, butter, milk, chocolate chips, eggs, salt, leavening. At first I thought they're just like every other Levain copycat but I've never seen a recipe with milk. After quite a few experiments I've made some nice thick cookies as well as chocolate chip biscuits and hockey pucks lol! I could really use some input before trying again. There is something about this particular crumb I can't seem to get. Any thoughts? Ideas regarding ingredient ratios?