Preparation

• Make the tart shell:In a medium bowl, combine the butter, sugar, zest, vanilla, and salt. Add the flour and mix just until well blended. If the dough seems too soft to work with, let it sit for a few minutes to firm up. Press the dough into a 9-1/2-inch fluted tart pan with a removable bottom. Start with the sides, making them about 1/4 inch thick, and then press the remaining dough evenly over the bottom, pressing well into the corners. Let rest at room temperature for 30 minutes or chill until ready to bake (you can make the crust up to 3 days ahead).

  Position a rack in the lower third of the oven and heat the oven to 350°F. Put the pan on a cookie sheet and bake until the crust is a deep golden brown, 20 to 25 minutes, checking after about 15 minutes to see if the dough has puffed. Press the dough down with the back of a fork and prick a few times if necessary.

  Meanwhile, make the filling:In a small saucepan bring the half-and-half, sugar, and salt to a simmer. Off the heat, add the chocolate and stir with a whisk until completely melted and smooth. Cover to keep warm.

  Just before the crust is ready, whisk the egg thoroughly into the chocolate mixture. When the crust is done, lower the oven temperature to 300°F. Pour the filling into the hot crust. Return the tart (still on the baking sheet) to the oven and bake until the filling is set around the edges but still jiggles a little in the center when you nudge the pan, 10 to 15 minutes. Cool on a rack.

  Serve at room temperature or slightly cool. Garnish each slice with crushedSalted Caramelized Pistachiosand a light sprinkling of fleur de sel.

  The tart is best on the day it’s made but may be refrigerated for 2 to 3 days. Once the tart is completely chilled, cover it, but make sure no plastic wrap touches the surface by first putting the tart pan in a larger cake pan. Or cover the tart with an overturned plate.