Oreo Thins Tiramisu crust: Place Oreos in a food processor or blender and process until they become fine crumbs. You should have around ¾ cup of crumbs. Transfer crumbs to a smallish bowl and pour in the melted butter. Stir until combined and all crumbs are buttered up. Place a layer of the buttered crumbs on the bottom of 6 small serving glasses (about 2 tablespoons of crumbs each). Set the glasses in the refrigerator while you prepare the cheesecake mousse.
Tiramisu cheesecake mousse: Using a handheld mixer (or stand mixer with paddle attachment), mix together the mascarpone and sugar until smooth and creamy (sugar should be dissolved!). Add the espresso (add more or less to suit your taste), and mix again on low speed until completely combined.
In a separate bowl, using the whisk attachment in your mixer, whisk the thickened cream to stiff peaks - be careful not to overbeat though, or it will become grainy.
Add about a third of the whipped cream to the mascarpone mixture and, with a silicone spatula, gently fold it in. Repeat two more times with the remaining ⅔ whipped cream, folding gently, until all is combined.
Grab the serving glasses from the refrigerator and fill them up with the tiramisu cheesecake mousse. Flatten the tops, or make pretty swirls, and place back in the refrigerator for a couple of hours to set (or eat straight away; I promise I won't tell anyone!).
Just before serving, dust the tops with some spare Oreo crumbs, if desired, and finish off with a whole Oreo Thins Tiramisu cookie. Serve immediately.