Prepare the cookie layer: In a medium bowl, combine cookie crumbs with melted butter and a pinch of salt until the mixture resembles wet sand. The butter helps the crumbs hold together, creating a more substantial base layer.
Melt the chocolate: Place chopped chocolate in a microwave-safe bowl and heat in 30-second intervals, stirring between each, until just melted (usually 1-2 minutes total). Be careful not to overheat as chocolate can burn quickly. Allow to cool slightly but remain fluid.
Whip 1½ cups of cream: In a large mixing bowl, beat 1½ cups of heavy cream with 2 tablespoons sugar and vanilla extract until soft peaks form. The cream should hold its shape but still have a slightly soft texture – don’t whip to stiff peaks or it won’t fold well.
Create the chocolate mousse: Gently fold one-third of the whipped cream into the melted chocolate to lighten it, then fold in the remaining whipped cream until no streaks remain. Work with a light hand to maintain as much air as possible in the mixture.
Assemble the base layers: Spoon about 2 tablespoons of cookie crumbs into the bottom of each parfait glass, pressing down gently. If using the coffee liqueur, drizzle 1-2 teaspoons over each cookie layer.
Add the chocolate layer: Spoon or pipe a generous layer of chocolate mousse (about 3-4 tablespoons) over the cookie base in each glass. The mousse should be thick enough to hold its shape.
Create alternating layers: Add another layer of cookie crumbs followed by another layer of chocolate mousse. Repeat until glasses are nearly full, ending with a chocolate layer.
Prepare the topping: Whip the remaining ½ cup of cream with 1 tablespoon sugar until stiff peaks form. This topping should be firmer than the cream used in the mousse.
Finish and garnish: Top each parfait with a dollop or piped swirl of whipped cream. Sprinkle with additional cookie crumbs, chocolate shavings, or fresh berries if desired.
Chill before serving: Refrigerate the parfaits for at least 1 hour (or up to 24 hours) to allow flavors to meld and the mousse to set properly. This resting time significantly improves the texture and flavor.