Step 1: Prepare the coffee. Brew strong coffee or espresso and let it cool completely. If using, stir in Kahlua or coffee liqueur. Set aside.
Step 2: Make the mascarpone cream base. In a medium heatproof bowl, whisk together egg yolks and granulated sugar. Place the bowl over a saucepan of simmering water (bain-marie), ensuring the bottom of the bowl does not touch the water. Whisk continuously for 5-7 minutes until the mixture is pale, thick, and reaches 160°F (71°C). Remove from heat, stir in vanilla extract, and let cool completely.
Step 3: Whip the heavy cream. In a large bowl, using an electric mixer, whip the very cold heavy cream and powdered sugar on medium-high speed until stiff peaks form. Be careful not to overbeat.
Step 4: Combine creams. Gently fold the cooled egg yolk mixture into the softened mascarpone cheese until smooth. Then, carefully fold in the whipped cream until just combined, being careful not to deflate the mixture.
Step 5: Assemble the tiramisu. Dip each graham cracker sheet briefly into the cooled coffee mixture (don't soak too long, they will get soggy). Arrange a single layer of coffee-dipped graham crackers at the bottom of an 8x8 inch (or similar) square dish or loaf pan.
Step 6: Layer. Spread half of the mascarpone cream mixture evenly over the graham cracker layer. Repeat with another layer of coffee-dipped graham crackers, followed by the remaining mascarpone cream.
Step 7: Chill. Cover the dish with plastic wrap and refrigerate for at least 4-6 hours, or preferably overnight, to allow the flavors to meld and the cake to set firmly.
Step 8: Serve. Before serving, dust generously with unsweetened cocoa powder. Slice and enjoy!