Prepare your chocolate by breaking up your chocolate bars and melting them slowly over low heat. You can do this one of a few ways: in a non-stick sauce pan, over a double boiler or in the microwave. I typically use a sauce pan or double boiler, I find chocolate melted in the microwave to be less shiny than chocolate melted slowly in a pan or double boiler. Note: I use chocolate bars, and not 'baking' or 'melting' chocolate(s), because I find that chocolate bars made to be consumed on their own (as opposed to those made to be used in baked goods) have more concentrated flavor. If you prefer baking chocolate, that's also fine to use here!

Once the chocolate is melted, pour it into a parchment-lined baking sheet and spread it out evenly with an offset spatula or the back of a metal spoon. The larger your baking sheet, the thinner your bark will be. I like my bark about half an inch thick, so I use a baking tray that measures 9x13.

While your chocolate is melting, melt the Nutella over low heat until it's easily spreadable.

Dollop the warm Nutella on the white chocolate in a random pattern, and then swirl it through the white chocolate in a figure-8 pattern using a butter knife.

Roughly chop the roasted hazelnuts and sprinkle them over the top of the bark.

Chill the bark in the fridge for at least an hour, until it's completely set and hard. Break apart into shards, or cut into bars and enjoy cold!