Mahleb, a spice that comes from a sour-cherry tree that flourishes in the Middle East and southern Europe, has an almondlike sweetness that is a great foil for dark chocolate. The mahleb can be omitted if you like.
Cook 1/4 cup sugar, the corn syrup, lemon juice, and 1 teaspoon water in a medium pot over medium heat until sugar melts and is deep amber, about 12 minutes. Immediately whisk in cream and milk, and whisk until incorporated, about 3 minutes. Bring mixture to a simmer, and add mahleb. Cover, remove pot from heat, and let mahleb steep 30 minutes.
Pour cream mixture through a fine sieve into a bowl, and discard mahleb. Return cream mixture to pot, and bring to a gentle simmer. Whisk together remaining 2 tablespoons sugar, the egg yolks, vanilla, and a pinch of salt in a heatproof bowl until pale. Gradually add cream mixture to eggs, whisking until incorporated. Return mixture to pot, and cook over medium heat, stirring, until thickened, about 3 minutes.
Immediately pour cream mixture over chocolate in a heatproof bowl. Let sit until softened, 1 to 2 minutes, then whisk until smooth.
Divide among eight 2-ounce serving cups, and chill until set, about 2 hours or up to 3 days. Before serving, lightly dust with cocoa powder.