1. In a large mixing bowl, combine the crab meat, 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.
2. 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.
3. Pour the sauce over the crab-meat mixture in the bowl and stir together with a spoon until the ingredients are well combined. Taste for seasoning.
4. 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.
5. Remove and spread the untoasted side of each round generously with the crab-meat mixture, mounding it slightly. These may be prepared in advance up to this point and then refrigerated.
6. Just before serving, place under a hot broiler for a minute or so until the canapes brown slightly. Serve very hot.
