In a large mixing bowl, combine the crabmeat, sherry, salt, pepper and dill and set aside. Melt the tablespoon of butter without browning it in a small, heavy saucepan, remove from the heat and stir in the flour. In a small bowl, beat the egg yolk with the cream and briskly stir this mixture into the butter-flour roux with a wire whisk. Return the pan to the heat and cook slowly, whisking constantly for a minute or two until the mixture thickens; do not let it boil. Pour the sauce over the crabmeat mixture in the bowl and stir together with a spoon until the ingredients are well combined. Taste for seasoning. Cut four rounds from each slice of bread, using a small cookie cutter or glass. Toast the bread rounds on one side only under a moderately hot broiler. Remove and spread the untoasted side of each round generously with the crabmeat mixture, mounding it slightly. These may be prepared in advance up to this point and then refrigerated. Just before serving, place under a hot broiler for a minute or so until the canap�s brown slightly. Serve very hot.