Step 1: Prepare the coffee. Brew strong espresso or coffee and let it cool completely. If using coffee liqueur, stir it into the cooled coffee.
Step 2: Make the mascarpone cream. In a large bowl, beat the heavy cream and vanilla extract until stiff peaks form. In a separate bowl, whisk together the mascarpone cheese, powdered sugar, and cocoa powder until smooth and creamy. Gently fold the whipped cream into the mascarpone mixture until just combined. Do not overmix.
Step 3: Dip the Oreos. Working quickly, dip each Oreo cookie briefly into the cooled coffee. 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.
Step 5: Add the first cream layer. Spread half of the mascarpone cream mixture evenly over the Oreo layer.
Step 6: Add a second Oreo layer. Arrange the remaining coffee-dipped Oreo cookies over the cream.
Step 7: Top with remaining cream. Spread the remaining mascarpone cream evenly over the second Oreo layer.
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.
Step 9: Garnish and serve. Before serving, dust generously with additional cocoa powder or finely crushed Oreo crumbs. Slice and enjoy your delicious Chocolate Oreo Tiramisu: Indulge in a Creamy Delight!