Preheat your oven to 350°F (175°C). In a mixing bowl, combine crushed chocolate cookies with melted butter until the mixture resembles wet sand. Line a muffin tin with cupcake liners and firmly press the cookie mixture into the bottom of each liner to form a sturdy base. Bake for 5 minutes, then set aside to cool completely.

In a saucepan over low heat, melt the semi-sweet chocolate chips and butter, stirring gently until smooth. Meanwhile, in a separate bowl, whisk together the heavy cream and cornstarch until fully combined. Gradually add this mixture to the melted chocolate, stirring continuously. Increase the heat to medium, and cook until the filling thickens and bubbles, about 5-7 minutes. Remove from heat and let cool slightly.

In a larger mixing bowl, blend the room-temperature mascarpone cheese with powdered sugar and vanilla extract until smooth and creamy. In another bowl, whip the heavy cream until stiff peaks form. Gently fold the whipped cream into the mascarpone mixture.

Start layering your Tiramisu Cookie Cups by spooning a layer of the chocolate filling into each cooled cookie base, followed by a layer of the mascarpone cream. Repeat the layers until you reach the top of the cupcake liners. Refrigerate the cups for about 10 minutes between each layer.

Once your cups are fully assembled, dust the top with unsweetened cocoa powder or add chocolate shavings. Cover and refrigerate them for at least 4 hours, or preferably overnight.