Cook ½ cup sugar in a small saucepan over medium heat until it begins to melt and turn brown around the edges. Using a heatproof rubber spatula, pull melted sugar at edges toward center of unmelted sugar. Continue cooking, pulling in melted sugar, until all the sugar is melted and caramel is uniformly dark brown (it should smell toasty but not burnt), 10–12 minutes total (if you still have undissolved lumps of sugar, stir off heat until they melt). Pour caramel into 8"-diameter cake pan to create an even, ⅛"–¼" deep layer. Let cool.
Bring 1 cup water to a simmer in a medium saucepan over medium-high heat. Whisk in cocoa powder until no lumps remain, then add cream, milk, vanilla bean and cinnamon stick and return to a simmer. Remove from heat and let steep 30 minutes.
Place a rack in middle of oven; preheat to 350°. Bring a large pot of water to a boil. Purée eggs with sugar and ¼ teaspoon salt in a blender until smooth and sugar is dissolved, about 1 minute. Strain chocolate mixture into egg mixture and blend at low speed to combine, discard solids. Skim foam off the top and transfer to prepared baking dish.
Line a roasting pan with a thick kitchen towel; this will keep baking dish from sliding and will insulate flan, preventing it from overcooking. Set dishes on towel and place roasting pan in oven. Pour boiling water into pan until it reaches the same level as flan in baking dishes. Bake flan until it looks set like Jell-O (it will wobble when gently shaken), 35–45 minutes. Transfer to a wire rack and let cool. Cover with plastic and chill until set and cold, at least 3 hours and up to 24.
Meanwhile, increase oven to 350°. Toss sesame seeds, syrup, and ¼ teaspoons salt in a small bowl. Scrape onto a silpat- or parchment-lined rimmed baking sheet and spread in a even layer. Bake brittle until sesame are golden brown and syrup is bubbly, 15–20 minutes. Let cool until hardened, then break up into small pieces; discard any dark brown, bitter pieces.
Run a small knife around edges of flan to loosen. Invert onto a platter and top with crushed candy.
Do Ahead: Flan can be made 1 day ahead. Cover and chill. Unmold and serve.