1. Preheat oven to 350ºF. Mist 6 6-oz. ramekins with cooking spray. Place a roasting pan in oven; pour in 1/2 inch of hot water. In a bowl, whisk flour, cocoa and baking powder.
2. Combine chocolate and butter in a microwave-safe bowl; cook on high, stirring once or twice, until almost melted, 1 to 2 minutes. Whisk until smooth.
3. In a bowl, with an electric mixer on medium-high speed, beat eggs, vanilla, sugar and salt until pale yellow, about 5 minutes. Beat in melted chocolate mixture. Stir in flour mixture. Spoon half of batter into ramekins. Drop 1 Tbsp. of Nutella into center of each. Cover with remaining batter.
4. Transfer ramekins to roasting pan. Bake until cakes have risen and are cracked on top, about 30 minutes. Let cool for 5 minutes. Serve warm.