Mix the cold coffee and the liquer in a large bowl. In batches, dip the cookies in the coffee mixture. You want them to be moist on the outside but still crunchy inside.
Beat the egg yolks with 1/2 of the sugar until the mixture is thick enough to form a long ribbon when you lift the beaters. Add the Mascarpone and beat for 2 – 3 more minutes. Set aside.
Beat the egg whites, adding the remaining sugar a bit at a time, until they form stiff peaks and have a glossy sheen, about four minutes. Gently fold the egg whites in the Mascarpone mixture, until the mixture is all the same color. Add the chopped semisweet chocolate and gently fold it into the mixture.
In a 9 x 17 glass baking dish, assemble the dessert. Layer the bottom of the pan with the soaked cookies. Top with a layer of the chocolate-Mascarpone mixture. Repeat the procedure to make one more layer. Using a flour sifter cover the top of the Tiramisu, sprinkle a thin layer of sweet cocoa powder. Place the Tiramisu in the refrigerator and let it rest for at least 5 hours, better if overnight.