Place the chopped dark chocolate in a heatproof bowl. Heat 1/2 cup heavy cream in a small saucepan over medium heat until it just begins to simmer, then pour over the chocolate. Let sit for 2 minutes, then stir until completely smooth and glossy. Stir in cocoa powder and espresso powder if using. Let cool to room temperature, about 15 minutes.

While chocolate cools, whip 2 cups of the cold heavy cream with powdered sugar and vanilla extract in a chilled bowl using an electric mixer on medium-high speed until stiff peaks form, about 3-4 minutes. Do not overbeat or cream will become grainy.

Once chocolate mixture has cooled, take 1 cup of the whipped cream and gently fold it into the chocolate in three additions using a rubber spatula. Use a folding motion rather than stirring to maintain airiness. This is your chocolate mousse layer.

In a separate bowl, whip the remaining 1/2 cup heavy cream with granulated sugar until soft peaks form, about 2 minutes. This will be your plain whipped cream layer.

Crush the chocolate cookies into varied textures — some fine crumbs, some chunky pieces. Set aside 1/4 cup for topping.

Begin layering in 6-8 serving glasses: Start with 2 tablespoons cookie crumbs, then 1/4 cup chocolate mousse, then 2 tablespoons plain whipped cream, then another tablespoon of cookie crumbs. Repeat layers once more, ending with a dollop of whipped cream.

Top each parfait with reserved cookie crumbs and optional garnishes like fresh berries or chocolate shavings. Cover loosely with plastic wrap and refrigerate for at least 1 hour before serving to allow layers to set and flavors to meld.

Serve chilled directly from the refrigerator. The parfaits are best enjoyed within 48 hours while the cookie layers still have some texture contrast.