To prepare the first layer: preheat the oven to 325 degrees F. Grease a 14"x10" glass baking dish with flour, and invert to shake out the excess. In a double boiler, melt the chocolate and butter over hot water. Cool. In a large bowl, beat the eggs and gradually add the sugar. Stir in the chocolate mixture, flour, salt, and vanilla. Beat for one minute. Pour into prepared baking dish. Bake for 25 minutes. Cool in the pan. To prepare the second layer: combine the butter, sugar, and Sherry. Stir until smooth, fold in the nuts. Spread over the first layer. Chill. To prepare the third layer: in a double boiler, melt the chocolate with the water and butter. Stir to combine. Drizzle over the layers. Freeze. Before serving, slice into two-inch bars.