Combine sugar, water, and corn syrup in saucepan and heat to amber.
Pour caramel into flan mold to coat base and set aside.
Preheat oven to 325°F (165°C).
Whisk eggs and yolks, then add milks.
Mix in cocoa powder, melted chocolate, vanilla, and salt.
Strain mixture and pour into caramel-lined mold.
Cover mold with foil, place in water bath.
Bake for 55–65 minutes until knife comes out clean.
Cool at room temp, then chill 4+ hours or overnight.
Run knife around edge, invert to unmold flan.