Bring the syrup back to a boil and cook over moderately high heat, stirring, until golden brown, 5 to 7 minutes. Remove from the heat and stir in the vanilla bean mixture, then the cashews and sesame seeds. Immediately scrape the brittle onto the prepared baking sheet and, working quickly, spread it in an even layer with an offset spatula. Lay another silicone baking mat or a sheet of sprayed parchment over the brittle and use a rolling pin to even it out. Remove the top baking mat and let the brittle cool completely, about 1 hour. Break the brittle into pieces before serving, storing or gifting.