Make the bars: Preheat the oven to 350º. In the bowl of a stand mixer fitted with the paddle attachment, beat the tahini and butter together to incorporate. Add the sugar and cream until light and fluffy, 2 minutes.
With the motor running, add the eggs, one at a time, until incorporated, followed by the vanilla and salt. Add the flour and mix until a thick batter forms. Transfer to a parchment-lined 9-by-13-inch pan and, using an offset spatula, spread into an even layer.
Bake until golden brown, 20 to 25 minutes, then set aside.
Meanwhile, make the ganache: In a medium bowl over a double boiler, combine the chocolate, butter and tahini. Whisk until the chocolate is completely melted and the mixture is smooth, 5 minutes.
Using an offset spatula, spread the ganache in an even layer over the warm cake. Sprinkle with sesame seeds and garnish with flaky sea salt. Let cool, then chill until firm, 1 hour, before cutting into squares or rectangles, and serving.