Use your favorite shortbread recipe and make a tart crust. Use a fork and prick the dough all over to prevent puffing. Bake at 350 F for ~15-20 minutes, until light brown.

Filling: Bring the sugar, cream of tartar and 1/3 cup water to a boil in a saucepan over medium low heat, covered. Do not stir. Cook and swirl the pot until the mixture is a deep amber color, 8-10 minutes. Remove the saucepan from the heat and stir in the butter, one piece at a time. It will splatter. Slowly stir in the cream. Transfer to a glass container and let cool until warm. Do not scrape out the pan into the glass container.

Pour the caramel into the cooled tart shell and chill for ~1 hour, until the caramel is firm.

Ganache: heat the chocolate, cream, and butter over a double boiler and stir until smooth, ~5 minutes. Let cool until can hold its shape but is still liquid. Pour the ganache over the caramel and use the back of spoon to create swirls if desired. Sprinkle with fleur de sel.