In another bowl, cream the butter and icing sugar with an electric mixer. Add the egg yolk and mix until smooth. With the machine running on low speed, mix in the dry ingredients until the dough comes together. Firmly press the dough along the bottom and up the sides of a 9-inch (23 cm) tart pan with removable bottom and 1-inch (2.5 cm) sides. Prick the dough with a fork. Refrigerate 30 minutes.
With the rack in the middle position, preheat the oven to 350°F (180°C).
Cover the dough with foil and fill with dry beans or ceramic pie weights.
Bake for 18 minutes. Remove the beans and foil. Continue to bake for 2 minutes to dry out the crust. Let cool for 30 minutes on a wire rack.
Caramel
In a pot, bring the sugar, water and corn syrup to a boil. Cook without stirring until an amber caramel forms. Remove from the heat. Gradually add the cream and butter. Watch out for splattering. Bring to a boil again and cook while stirring until a candy thermometer reads 234°F (112°C). Add the fleur de sel.
Pour the caramel into the cooled crust. Refrigerate for 1 hour.
Ganache
Place the chocolate in a bowl.
In a small pot or in the microwave oven bring the cream and butter to a boil. Pour the hot cream over the chocolate. Let sit for 1 minute without stirring. Whisk until smooth. Spread the ganache over the caramel. Refrigerate for 1 hour.
When ready to serve, sprinkle fleur de sel over the tart. The tart will keep for 5 days, covered, in the refrigerator.
