Tartine's Lemon Tart

This was a rich, deliciously tart, crispy dessert that we all really enjoyed.  I found this recipe in Tartine, and we liked that the tart wasn't too sour, as sometimes lemon tarts can be.  We all gave this a perfect 10, and the calories per serving were around 450, depending on the tart shell used.

1 recipe pate sablee (here is a link to one from Martha Stewart, I used the one in Tartine), cooked and cooled

1/2 c plus 2 T lemon juice
3 whole large eggs
1 egg yolk
3/4 c sugar
pinch of salt
2 sticks/1 cup butter

In a double boiler, combine the lemon juice, whole eggs, egg yolk, sugar, and salt.  Whisk together constantly, allowing the mixture to become very thick, which could take quite a while.  Remove the bowl from the boiler and allow the mixture to cool a little.  Cut the butter into small pieces, and then place the butter piece by piece in the lemon mixture, and using an immersion blender, blend up the butter.  The cream will be opaque and quite thick.  Pour it into the tart pan, cover and let cool completely.  Serve with whipped cream.