Step 1: Prepare the coffee. Brew strong espresso or coffee (about 2 cups) and let it cool completely. If using coffee liqueur, stir it into the cooled coffee for an added layer of sophisticated flavor. Set aside in a shallow dish for easy dipping.
Step 2: Make the mascarpone cream. In a large mixing bowl, beat 2 cups of chilled heavy cream and 1 teaspoon of vanilla extract using an electric mixer until stiff peaks form. In a separate bowl, whisk together 16 oz (450g) mascarpone cheese, 1/2 cup (60g) powdered sugar, and 1/4 cup (25g) unsweetened cocoa powder until smooth and creamy. Gently fold the whipped cream into the mascarpone mixture using a spatula until just combined.
Step 3: Dip the Oreos. Working quickly, dip each Oreo cookie briefly (just a second or two per side) into the cooled coffee mixture. You want them moistened but not completely saturated.
Step 4: Assemble the first layer. Arrange half of the coffee-dipped Oreo cookies in a single layer at the bottom of a 9x13-inch (23x33 cm) baking dish, fitting them snugly to create a solid base.
Step 5: Add the first cream layer. Spread half of the mascarpone cream mixture evenly over the Oreo layer, using an offset spatula to ensure a smooth, consistent layer.
Step 6: Add a second Oreo layer. Arrange the remaining coffee-dipped Oreo cookies over the first cream layer, ensuring good coverage.
Step 7: Top with remaining cream. Spread the remaining mascarpone cream evenly over the second Oreo layer, creating a beautiful, smooth finish.
Step 8: Chill the tiramisu. Cover the dish tightly with plastic wrap and refrigerate for at least 6 hours, or preferably overnight, to allow the flavors to meld and the tiramisu to set properly.
Step 9: Garnish and serve. Before serving, dust generously with additional unsweetened cocoa powder or finely crushed Oreo crumbs. Slice into generous portions and enjoy your delicious Chocolate Oreo Tiramisu.