Step 1Sweeten the hot coffee or espresso with 1 1/2 tablespoons sugar.
Step 2Coarsely chop the unsweetened chocolate and 4 ounces of the semi-sweet chocolate. Melt in a double boiler or a bowl over a pot of simmering water. Add the sweetened coffee a little at a time, stirring continuously. Once the coffee and chocolate are combined, remove from the heat and cool. The mixture may be slightly lumpy; this is OK. Set aside.
Step 3Make zabaglione by whisking together the egg yolks and one-fourth cup plus 2 tablespoons sugar. Add three-fourths cup Marsala. Place the bowl over a pot of simmering water and cook until the mixture thickens, about 6 minutes. Remove from the heat and cool.
Step 4Whip the cream together with one-half tablespoon sugar.
Step 5Fold the cooled zabaglione into the whipped cream.
Step 6Line the bottom of a 9-by-5-inch loaf pan with waxed paper. Fill the pan one-third full with the zabaglione and whipped-cream mixture, smoothing the top with a spatula.
Step 7Pour the remaining 1 cup Marsala into a small bowl. Briefly dip each cookie in the Marsala just until moistened, a few seconds per side (don't let them get soggy). Place the moistened cookies on top of the mixture in the loaf pan, lining the cookies up side by side and as close to each other as possible.
Step 8Gently fold the chocolate and coffee mixture into the remaining zabaglione mixture, combining thoroughly. Spoon the chocolate mixture over the cookie layer, covering evenly to the top of pan. Smooth the top with a spatula.
Step 9Freeze the semifreddo for at least 3 to 4 hours. After 4 hours, cover it with plastic wrap.
Step 10To serve, heat a knife in hot water and run it around the edge of the frozen semifreddo to loosen it from the pan. If necessary, briefly place the semifreddo in a pan of hot water to loosen. Turn the pan upside down on large flat plate, lift the pan off and peel the wax paper from the dessert. Slice and place on individual serving plates. Shave the remaining semi-sweet chocolate on each to garnish.