Step 1: Prepare the Oreo Crust
Start by crushing the Oreos. You can do this by placing the cookies in a plastic bag and rolling over them with a rolling pin or by using a food processor. The goal is to get fine crumbs, but larger pieces can add a bit of texture to the crust, which is perfectly fine! Once the cookies are crushed, set them aside.

Step 2: Make the Chocolate Mousse
Begin by melting the dark chocolate. You can do this in the microwave in short bursts (20-30 seconds at a time) or use the bain-marie method. The bain-marie method involves placing a heatproof bowl over a pot of simmering water, allowing the chocolate to gently melt. Stir the chocolate frequently to prevent it from burning. Once the chocolate is melted and smooth, set it aside to cool slightly.

Next, in a large mixing bowl, combine the heavy cream, sifted cocoa powder, powdered sugar, and vanilla extract. Use an electric hand mixer to whip this mixture on high speed until stiff peaks form. This step is crucial because it gives the mousse its airy and light texture. The whipped cream should form peaks that stand upright when you lift the mixer.

Once the whipped cream is ready, gently fold in the melted chocolate with a spatula. This ensures the mousse remains light and fluffy while incorporating the rich chocolate flavor. Continue mixing until fully combined.

Step 3: Make the Whipped Cream
In a separate bowl, combine the heavy cream, powdered sugar, and vanilla extract. Using your electric hand mixer, whip until stiff peaks form. This process usually takes only a couple of minutes, but be careful not to overwhip, or your cream may become too stiff and turn into butter.

Step 4: Assemble the Parfaits
Now it’s time to layer your parfaits! Begin by adding a tablespoon of crushed Oreos to the bottom of each glass or serving dish. Use a spoon or your hands to lightly press the crumbs into the base, forming a solid foundation.

Next, pipe or spoon a layer of chocolate mousse over the Oreo crumbs. Follow this with a layer of whipped cream, and then repeat the layers: more Oreo crumbs, followed by mousse and whipped cream. For the final layer, top with extra Oreo crumbs for decoration and texture.

Once all the parfaits are assembled, cover each glass with plastic wrap and refrigerate for at least 3 hours, or until the mousse has set and the layers are firm. The chilling time allows the flavors to meld together and ensures the parfaits have a smooth, creamy texture.