Start by crushing the 5 Oreo cookies into fine crumbs. You can do this by placing them in a zip-top bag and rolling over them with a rolling pin or using a food processor for a quick solution. Set aside.
In a microwave-safe bowl, melt the 2 ounces of semi-sweet chocolate. Heat in short increments, stirring in between until smooth. Allow it to cool slightly.
In a mixing bowl, combine 1 cup of heavy cream and 1 tablespoon of granulated sugar. Using an electric mixer, whip until soft peaks form. Be careful not to over-whip; you want it to remain fluffy.
In another bowl, combine the 8 ounces of mascarpone cheese, ½ cup granulated sugar, 1 teaspoon vanilla extract, and 2 tablespoons of unsweetened cocoa powder. Mix until smooth and creamy.
Fold the melted chocolate into the mascarpone mixture until well incorporated. Then, gently fold in the whipped cream, being careful to maintain the light texture.
In your trifle cups or small glasses, start layering by adding a spoonful of crushed Oreos at the bottom. Follow with a layer of the chocolate mascarpone mixture. Repeat the layers until the cups are filled, finishing with a layer of the mascarpone mixture on top.
Cover the trifles with plastic wrap and refrigerate for at least 2 hours, allowing the flavors to meld together. Serve chilled, garnished with extra cocoa powder or crushed Oreos if desired.