Preheat oven to 350 degrees F. Spray a 9x9 inch baking dish with cooking spray and line with parchment paper.
Place black sesame seeds in a food processor and pulse until coarsely ground. Combine seeds with graham cracker crumbs, brown sugar, and melted butter in a medium bowl. Mix until well combined and transfer to prepared baking dish. Press down on crust mixture to create one even layer. Bake for 12 minutes or until golden brown. Remove from oven and let cool completely.
Meanwhile, make chocolate filling. Heat milk in a medium saucepot over medium heat until it comes to a bare simmer.
Combine sugar with cocoa powder, cornstarch, salt, and egg yolks, mixing until yolks are broken. The mixture will be dry.
Slowly add milk to yolk mixture, stirring constantly. Pour chocolate filling back into pot and bring to a simmer over low heat. Continue to cook until thickened, about 5 minutes, stirring frequently. Remove from heat and add vanilla. Pour chocolate filling into crust and spread in an even layer.
Combine sugar with light corn syrup with water in a medium-heavy bottom saucepot. Stir to combine and bring to a simmer over medium heat. Continue to simmer until a candy thermometer registers 240 degrees F.
While the sugar mixture is simmering, pour egg whites and cream of tartar into the bowl of a stand mixer. Whip egg whites on medium speed until soft peaks form.
While the mixer is running, slowly add sugar syrup directly into the egg whites until you've added all of the sugar. Add matcha powder and continue to whip egg whites until stiff peaks form, about 6-7 minutes.
Top chocolate filling with matcha marshmallow fluff, creating peaks with the back of a spoon if desired. Transfer dessert bars to the fridge and chill for at least 1 hour.
Right before serving, use a kitchen blowtorch to torch the tops if desired. Slice and serve.