Layer a 6″x4″ baking dish with foil or parchment paper, leaving some hanging over the sides. Generously butter and set aside.
Heat the brown sugar and heavy cream over medium-low heat until it just starts to boil, then turn the heat down to low. Attach a candy thermometer to the side of the pan, and stir the sugar until the thermometer reads 240*F, or soft-ball stage. This will take about 10-15 minutes. Once the temperature is 240*, remove the sugar from the heat and add the butter, but do not stir the butter in. Let the mixture sit until the temperature is at least 140*F- about 20 to 30 minutes.
When the temp is low enough, begin mixing with a hand mixer on low. Mix, about 5-7 minutes, until the candy has thickened and any gloss has started to fade.
Pour the candy into the buttered dish and smooth out any large bumps. Layer the crushed hazelnuts on top evenly, and set in the fridge at least 2 hours, or until firm. Remove from the dish by pulling up on the overhang side of parchment or foil, and cut into squares. Store tightly wrapped and chilled.