Spread the hazelnuts on a baking sheet and bake for about 10 minutes.
While they’re in the oven, get a clean towel and lay it on the counter.
Remove the hazelnuts and let cool for a minute or two. Transfer to the towel and rub the hazelnuts together until skins are removed. You may have to do this in batches.
Remove the hazelnuts — it’s okay if there are some with a bit of skin left — and transfer to a saucepan.
Add both sugars, water, butter, and salt. Turn the heat to medium low and let the sugar melt. Stir the hazelnuts by pushing them inward toward the center with a spatula. Attach your candy thermometer if using.
While the hazelnuts are cooking, grab a sheet pan and line with parchment.
Let the hazelnuts cook over low heat until the thermometer reaches 290 degrees. If the sugar looks granulated, turn the heat off and add a tablespoon of water and stir.
When you’ve reached 290 degrees, turn off heat and stir in the vanilla.
Pour the hazelnuts on the sheet pan, spreading them out as much as you can.
Let them cool completely, and break up any that still remain together.
Transfer these to a clean parchment lined sheet pan and pop in the freezer for about 30 minutes.
Melt the chocolate in the microwave, stirring every 20 seconds until smooth.
Remove the hazelnuts from the freezer. Drizzle with the melted chocolate and immediately sprinkle with sea salt.