Step 1: Prepare the coffee base. In a shallow dish, combine the cooled strong coffee (or espresso) and granulated sugar. Stir until the sugar is fully dissolved. If using, add the coffee liqueur here. Set aside.
Step 2: Prepare the mascarpone cream. In a large mixing bowl, beat the softened mascarpone cheese with an electric mixer on medium speed until smooth. In a separate cold bowl, whip the cold heavy cream with vanilla extract and sifted powdered sugar until stiff peaks form. Gently fold the whipped cream into the mascarpone cheese until just combined, being careful not to overmix. A light touch ensures the cream remains airy and luxurious.
Step 3: Crush some Oreos. Take about 8-10 Oreo cookies and finely crush them in a food processor or by placing them in a Ziploc bag and crushing with a rolling pin. Set aside for layering and garnish.
Step 4: Begin assembling the tiramisu. Dip each remaining Oreo cookie (not the crushed ones) quickly into the coffee mixture. Do not let them soak too long, or they will become soggy. A quick dunk, just a second or two per side, is all they need.
Step 5: Arrange the first layer. Arrange a single layer of coffee-dipped Oreos at the bottom of a 9x13 inch (or similar sized) baking dish. Make sure they cover the bottom as much as possible.
Step 6: Layer the cream. Spread half of the mascarpone cream evenly over the first layer of Oreos. Use an offset spatula for a smooth finish. Sprinkle half of the reserved crushed Oreos over the cream layer.
Step 7: Repeat the layers. Add another layer of coffee-dipped Oreo cookies over the cream. Top with the remaining mascarpone cream, spreading it smoothly to cover all cookies.
Step 8: Finish and chill. Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow the flavors to meld and the tiramisu to set. The longer it chills, the better the flavors develop and the firmer the structure becomes.
Step 9: Serve. Before serving, dust generously with unsweetened cocoa powder and sprinkle with the remaining crushed Oreo cookies. Slice and enjoy this incredible delightful dessert!