Toss your chocolate sandwich cookies into a ziplock bag and seal it tightly. Use a rolling pin or your hands to crush them into small crumbs. You want texture here, not dust, so leave some larger pieces for crunch.
Add your chopped chocolate and half a cup of the heavy cream into a microwave-safe bowl. Heat in 30-second bursts, stirring between each round, until it turns smooth and glossy. This should only take about a minute or two total.
Pour the remaining heavy cream, powdered sugar, and vanilla extract into a mixing bowl. Whisk on low for a minute, then bump the speed up to medium until you hit soft, stable peaks. This usually takes four to five minutes.
Scoop out half of the whipped cream and set it aside for later. With the mixer running on low, slowly add the cooled chocolate ganache and cocoa powder into the remaining cream. Mix until it's glossy and fully blended into a rich mousse.
Grab your glasses and start with a spoonful of cookie crumbs at the bottom. Add a layer of chocolate mousse, then a layer of whipped cream, and repeat until you reach the top. Finish with a final sprinkle of cookie crumbs for that classic parfait look.
Cover each glass loosely with plastic wrap and slide them into the fridge. Let them set for at least three to four hours, though I usually leave mine overnight for the best texture and flavor.