Heat the oven to 160°C. Arrange six 150 ml ramekins (or one 20 cm / 8-inch ring mold) inside a deep roasting pan.
Caramel
Combine the sugar, water, and lemon juice in a small, heavy saucepan over medium heat. Swirl the pan occasionally — do not stir — until the sugar dissolves, bubbles, and turns a rich amber color (about 6–8 minutes).
Immediately divide the hot caramel among the ramekins, swirling each quickly to coat the bottom. Set aside to harden.
Custard Base
In a small saucepan, bring the red vermouth, orange peel, and cinnamon stick to a gentle simmer for 3 minutes to mellow the raw alcohol and concentrate the botanicals.
In a separate pot, heat the whole milk until steaming (do not boil).
Discard the peel and cinnamon stick, then whisk the warm vermouth into the milk. Remove from heat.
In a large bowl, whisk the whole eggs, egg yolks, and granulated sugar together until just blended. Avoid whisking vigorously to prevent excess air bubbles.
Slowly pour the warm milk-and-vermouth mixture into the eggs in a steady stream, whisking constantly.
Pour the custard through a fine-mesh sieve into a measuring pitcher for a completely smooth texture.
Pour the strained custard into the caramel-coated ramekins. Pour boiling water into the roasting pan until it reaches halfway up the sides of the ramekins.
Bake for 40 to 45 minutes (or 50–55 minutes for a single large mold) until the edges are set but the centers still have a gentle jiggle.
Remove the ramekins from the water bath and cool to room temperature on a wire rack. Cover and refrigerate for at least 4 hours, preferably overnight, so the caramel softens and turns to syrup.
Serving
Whip the cold heavy cream and powdered sugar until soft peaks form.
Run a thin, flexible knife around the edge of each ramekin and invert onto serving plates.
Dollop a generous spoonful of cream beside or over each flan, then dust heavily with the bitter cocoa powder right before serving.
Notes
Custard sugar is adjusted down to account for the sugar in the vermouth.
Pairs herbal, bittersweet botanicals with a silky egg custard, lightly whipped cream, and dark bitter cocoa.