In a large bowl, whisk the Mascarpone and lemon zest. Set aside.
Whisk the egg yolks, liquor and ½ cup of granulated sugar in a large stainless-steel bowl. Set the bowl over a large saucepan filled with 1 inch of barely simmering water. Using a hand-held electric mixer on low speed or a whisk, beat the egg-yolk mixture until it is hot and the mixture forms a ribbon when the beaters are lifted, 8-10 minutes. Don't cook the zabaglione for too long, or it will curdle. Scrape the zabaglione into the Mascarpone mixture and whisk until smooth.
In a clean bowl add the cream and using an electric mixer or a whisk, whisk while gradually adding the remaining ½ cup of sugar, until soft peaks have formed, about 5 minutes. Gently fold the cream into the egg mixture until well combined. Refrigerate the mixture while you prepare the rest of the tiramisu.
Place the espresso in a shallow bowl. Dip the rounded side of each biscuit into the espresso, careful not to let it sit for too long or the biscuit will fall apart.
In a 9" x 13” baking pan, place 24 biscuits, side by side to evenly fill the bottom of the pan. Top with half the cream and a light dusting of cocoa powder. Place the remaining biscuits on top and cover with remaining cream. Cover with a light dusting of cocoa powder.
Divide into 8 portions, dusting with extra cocoa once plated. Sprinkle over shaved chocolate. Serve immediately.
