Warm Cocoa Pudding with Candied Pecans

Preheat oven to 325°F. Whisk egg white in a medium bowl until frothy. Whisk in sugar. Add pecans; toss until coated. Spread out on a parchment-lined rimmed baking sheet and bake, tossing every 10 minutes, until browned and dry, 25–30 minutes. Let cool.
Bring cream, milk, butter, and vanilla to a simmer in a medium saucepan over medium heat. Meanwhile, whisk eggs, sugar, cornstarch, cocoa powder, and salt in a medium heatproof bowl to combine.
As soon as cream mixture reaches a simmer, remove from heat and, whisking constantly, very gradually add one-third of cream mixture to egg mixture. Whisk tempered egg mixture into cream mixture in saucepan. Cook over medium heat, whisking constantly, until mixture thickens and is bubbling, about 2 minutes. Remove from heat and add chopped chocolate; whisk until chocolate is melted and pudding is smooth. Serve topped with candied pecans.
Pecans can be candied 1 week ahead. Store airtight at room temperature. Pudding can be made 2 days ahead. Press plastic wrap directly onto surface and chill. Reheat gently in a heatproof bowl set over a pan of simmering water before serving.