Melt chocolate and butter: Melt dark couverture and butter together, either in a microwave or over a double boiler.
Mix wet ingredients: Whisk together eggs, yolk, and granulated sugar, then incorporate the melted chocolate mixture.
Add dry ingredients: Fold in flour, cocoa, salt, and chunks of milk and white chocolate.
Prepare for cooking: Transfer the batter into a buttered mold lined with parchment paper. Place in a pressure cooker with water and cover tightly with aluminum foil.
Cook: Cook in the pressure cooker for 30 minutes. Let cool before cutting into pieces.
Serve: Serve brownie pieces drizzled with melted milk and white chocolate.