Nutella Bark

Line a large rimmed baking sheet with parchment paper.

Put chopped dark chocolate in a shallow, medium heatproof bowl. In a medium saucepan, bring 1/2 in. water to a bare simmer. Set bowl over pan and heat chocolate, stirring often, until melted and registers about 115° on a digital or instant-read thermometer, 3 to 7 minutes.

Remove bowl from water and dry underside. Let chocolate cool (at first, it will rise about 5°), stirring constantly, to 95°, 5 to 12 minutes. Add large dark chocolate pieces and stir constantly until chocolate cools to 88° to 90°, 2 to 3 minutes. Spoon out unmelted dark chocolate and save for other uses.

Pour melted dark chocolate onto center of parchment and spread with an offset spatula or flexible scraper to about 1 in. from pan edges. Let cool at least 30 minutes.

Microwave Nutella in a 1-cup glass measuring cup until 95° to 100°, 30 seconds. Stir, then cover tightly with plastic wrap and set on a folded towel to keep warm; set aside.

Put chopped white chocolate in a clean, shallow, medium heatproof bowl. Return water in saucepan to a bare simmer. Set bowl over pan and heat chocolate, stirring often, until melted and registers about 110° on a digital or instant-read thermometer, 3 to 4 minutes.

Remove bowl from water and dry underside. Let chocolate cool (at first, it will rise about 5°), stirring constantly, to 95°, 5 to 12 minutes. Add large white chocolate pieces and stir constantly until chocolate cools to 86° to 88°, 2 to 3 minutes. Spoon out unmelted white chocolate and save for other uses. Stir Nutella into melted chocolate, then spread over dark chocolate. Sprinkle with hazelnuts.

Set pan on a cooling rack and let stand at room temperature until chocolate is firm, about 7 hours. Cut or break bark into pieces.

Make ahead: Up to 1 month, airtight at room temperature.