Heat the oven to 400 degrees F/200 degrees C.
Put the butter, sugar, salt, 2 to 3 tablespoons cold water and egg in a bowl and mix well. Add the flour and mix quickly only until it holds together. Do not overwork. Roll the dough out to 1/16 of an inch/2 mm thick. Cut into squares. Sprinkle over some fleur de sel and the topping(s) of your choice on each. Transfer to a baking sheet. Bake until puffed, crisp and golden, about 10 minutes.
Recipe courtesy of Laura Calder