01 -
Preheat oven to 190°C (375°F). Combine flour, powdered sugar and butter cubes in a small bowl. Using a pastry blender, cut butter into flour until mixture is pebbly and holds together when gathered. With floured fingers, press the dough across the bottom and sides of a 23cm (9-inch) springform pan or removable bottom tart pan. Bake until golden brown and crisp, 18-20 minutes. Remove and set aside to cool.
02 -
Preheat oven to 175°C (350°F). In a bowl, stir together the sugar, egg whites, and coconut until completely combined. Press mixture into a 23cm (9-inch) tart pan. Bake until golden, 12-15 minutes. Note that the tart will become more golden after it is baked with the filling.
03 -
Place cream in a small heavy saucepan over medium-low heat until beginning to steam. Do not let it boil. Add the chocolate and whisk gently until melted and the mixture is smooth and glossy, 2-3 minutes. Whisk in the egg, remove from heat, and set mixture aside to cool and thicken slightly, about 5 minutes.
04 -
Pour the cooled chocolate mixture into the cooled crust and bake at 190°C (375°F) for about 15 minutes. Center should still be slightly moist when a knife is inserted. Let cool until no longer piping hot.
05 -
Place sugar and vanilla in a medium-sized heavy pot over medium heat. Cook, stirring occasionally, until sugar is very soft, fragrant and beginning to melt, about 5 minutes. Add espresso and butter, stirring until well blended, about 2 minutes. Add cream and kosher salt, bringing to a simmer. Continue stirring until sauce reaches the consistency of thick honey, about 15 minutes. Remove from heat and discard vanilla pod if used. If sauce becomes too thick upon standing, stir in additional heavy cream and heat in microwave on low until pourable.
06 -
Pour butterscotch sauce over chocolate filling and spread evenly. Note that runny butterscotch will thicken when refrigerated. Sprinkle with salt flakes if desired. Cover loosely and refrigerate for at least 2 hours and up to 3 days. For enhanced flavor, allow tart to come to room temperature before serving.