Preheat oven to 350 F and butter an 8 or 9 inch square baking dish. Line the bottom with two overlapping sheets of parchment paper, leaving at least a two inch overhang on all sides.

In the bowl of a food processor fitted with blade, pulse to combine the flour, brown sugar, powdered sugar and salt. Add the sesame seeds and pulse once or twice to combine. Add the butter and pulse until the butter is about the size of peas (some larger pieces are okay). Add the egg yolk and pulse until just incorporated then add the water.

Turn dough out into the prepared pan and use your hands to press the mixture into the base of the pan. Bake for 20-23 minutes or until lightly golden brown. Remove from oven and place the pan on a cooling rack to cool.

Combine the water and sugar in a medium saucepan set to medium heat. Using a light colored sauce pan is preferable so that you can better see when the sugar begins to change color. Bring the mixture to a boil and leave on a low boil over medium heat for 11-13 minutes until it turns deep reddish orange.

Remove pan from heat and whisk in the butter and cream. Once the butter has melted, stir in the salt and tahini. Pour the tahini caramel sauce over the cooled shortbread. Use an offset spatula, if needed, to help spread the caramel evenly. Allow to cool for at least an hour before making and adding the chocolate layer over top.

Place the chocolate and heavy cream in a microwave-safe bowl. Microwave in 25 second bursts, stirring well in between. Do this until the chocolate is just melted, being careful not to burn/overcook the chocolate. Pour the melted chocolate over the tahini caramel and use an offset spatula to spread it. Sprinkle with sea salt and or toasted sesame seeds, if you like. Cover and place in the refrigerator for 1- 2 hours or until chocolate has set.