With a sharp, serrated knife, slice the end crusts off the pound cake and level the top if it is rounded. Cut the cake horizontally into 1/2 to 3/4 inch thick slabs.
Rub the ricotta through a coarse sieve into a bowl with a wodden spoon and beat it with a rotary or electric beater until it is smooth. Beating constantly, add the cream, siugar and Strega.
With a rubber spatula, fold in the chopped candied fruit and chocolate. Center the bottom slab of the cake on a flat plate and spread it generously with the ricotta mixture.
Carefully place another slab of cake on top, keeping the sides and ends even, and spread some more ricotta. Repeat until all the cake slabs are reassembled and the filling has been used up- ending with a plain slice of cake on top.
Gently press the loaf together to make it as compact as possible. Do not worry if it feels wobbly; chilling firms the loaf. Refrigerate the cassata for about 2 hours, or until the ricotta is firm.
Melt 12 oz. of chocolate with the coffee in a small heavy saucepan over low heat, stirring constantly until the chocolate has completely dissolved.
Remove the pan from the heat and beat in the chilled butter, 1 piece at a time. Continue beating until the mixture is smooth. Then chill this frosting until it thickens to spreading consistency.
With a small metal spatula, spread the frosting evenly over the top, sides and ends of the cassata, swirling it as decoratively as you can.
Cover loosely with plastic wrap, wax paper or foil and let cassata "ripen" in the refrigerator for at least 24 hours before serving it.