In the vast world of Beijing snacks, few dishes are as proudly local – or as misunderstood by outsiders – as Fried Ma Tofu (炒麻豆腐, 灰白、红、绿三色相间), this unassuming little dish doesn’t look like much. But to old-school Beijingers, it’s nothing less than a “unique treasure” of the capital’s traditional Han cuisine – a taste…