Chocolate Semifreddo

Make the crust:
1 Heat oven to 350 ( 175 C) degrees. Pulse the cookies, sugar and salt in a food processor until fine crumbs form. Add the butter and pulse until the mixture is moistened and holds together when you pinch it. Press about 2 tablespoons crumbs onto the bottoms of each of four 4 x 1 ¾-inch (or use one 8-inch springform pan). Place the pans on a baking sheet and bake 5 minutes.
Make the filling
1 Put 2 tablespoons of the sugar and ¼ cup water in a small saucepan over medium heat until the water simmers and sugar is dissolved; add the chocolate and cocoa off the heat and stir until completely smooth. Transfer to a large bowl and cool 10 minutes.
2 Beat the egg yolks and remaining ¼ cup sugar with an electric mixer and the whisk attachment on medium-high speed until light colored and thickened enough to form a ribbon, about 5 minutes. Fold the egg yolk mixture into the cooled chocolate until evenly blended, then transfer to another bowl.
3 Wash and dry the mixer bowl. Add the cream and mascarpone and beat on high speed until soft peaks form. Gently fold the cream into the chocolate mixture.
Assemble
1 Divide the mixture among the 4 pans (or one 8 inch pan, scrape it all in). Cover pans with plastic wrap and freeze at least 6 hours or overnight.
2 Remove semifreddo from the freezer 15-30 minutes before serving, depending on how firm it is. Unmold and serve with some whipped cream and chocolate shavings over the top, if you like.