Place the egg yolk in a mixing bowl and add the vinegar, mustard, tomato paste, salt and pepper to taste and cayenne. Gradually add the oil while beating with a whisk. Beat in the lemon juice, tarragon and brandy.
Add the crabmeat to the dressing and fold in. You can do this in advance and chill an hour or so if desired.
When ready to serve, stack the lettuce leaves and cut into fine shreds with a sharp knife. Fold this into the salad and serve immediately.