Preheat the oven to 350°F (175°C) and line a muffin tin with cupcake liners. Combine the crushed chocolate cookies and melted butter in a bowl, mixing until the texture resembles wet sand. Press the mixture firmly into the bottom of each liner to create a compact base. Bake for 5 minutes, then allow to cool completely.

In a small saucepan, combine the cooled espresso, coffee liqueur (if using), chocolate chips, and butter. Melt over low heat, stirring until smooth. In a separate bowl, whisk together the heavy cream and cornstarch until fully combined and lump-free. Gradually add this mixture to the saucepan, stirring constantly over medium heat until it thickens to a pudding-like consistency. Remove from heat and cool to room temperature.

In a bowl, beat the mascarpone, powdered sugar, and vanilla extract until smooth and creamy. In a separate bowl, whip the cold heavy cream until stiff peaks form. Gently fold the whipped cream into the mascarpone mixture until fully combined.

Begin layering by adding 1-2 tablespoons of either the mascarpone cream or coffee-chocolate mixture onto the cooled cookie base. Smooth the layer, then add an alternate layer of the opposite filling. Continue layering until the liners are nearly full, chilling the cups for 10 minutes between layers to maintain stability and definition.

Generously sift unsweetened cocoa powder over the top of each assembled cup. For added garnish, sprinkle chocolate shavings or mini chocolate chips if desired. Refrigerate the cups for at least 4 hours or overnight to allow the layers to set and flavors to meld beautifully before serving.