Like a créme brulee, this dessert has crunchy caramelized sugar on top. But a silky lemon filling is below, chilled in a crisp tart shell made with paté sucree.
Place sugar and lemon in a large stainless-steel bowl. Push yolks through a sieve into bowl, and whisk to combine.
Set bowl over a pot of simmering water, and stir slowly until mixture thickens, 15 to 20 minutes. Cook 5 minutes longer.
Remove bowl from heat, and stir in zest. Stir in butter piece by piece until completely melted. Pour into cooled tart shell. Chill until firm, at least 1 hour.
Preheat broiler. Remove outer ring from tart pan, and place pan on a large cookie sheet. Place outer ring upside down on top to protect pastry from burning.
Sift sugar evenly over top, and place under broiler. Watch carefully; remove tart when top is evenly browned.