Preheat oven to 350º. Line a 13x9” baking pan with parchment paper. Beat butter until creamed, approx. 3 min. Add sugars and beat until fluffy, 1 min more. Add egg yolk and vanilla, scraping down sides. Add flour gradually and sprinkle in salt. Do not overmix. Dough will be loose. Turn out into the pan and press evenly with hands until dough covers entire pan. Dock (poke) dough with a fork every ½” or so. Bake 25 min, until edges are lightly browned. Cool before adding caramel layer.
In medium saucepan over medium heat, melt butter and brown sugar, stirring constantly, until sugar dissolves, 3 min. Whisk in cream and salt. Bring to a boil and whisk constantly until thick enough to coat the back of a spoon, 5 min. Remove from heat and stir in tahini and vanilla. Pour over shortbread and chill completely before adding chocolate ganache.
Combine chocolate and cream in a glass bowl and microwave in 30-sec intervals until completely melted together, stirring to combine. Add vanilla and pour evenly over butterscotch/shortbread, lifting and turning pan to spread chocolate. Chill completely. Cut into squares and top with scant pinch of flake salt and sesame seeds.