Yield: makes 1-9" tart or 4-5" tartlets
Preheat oven to 350 degrees F. Toast hazelnuts in oven for 8-12 minute until the skins begin to separate from the nut. Rub the warm hazelnuts in a clean dish towel to remove the skins.
In a food processor, grind 2/3 cup of the hazelnuts with graham crackers, cocoa powder, sugar and salt. Add melted butter and pulse until moisture is distributed. Transfer mixture to a 9-inch removable bottom fluted tart pan.
Press mixture into the bottom and up the sides of the tart pan. Use the bottom of a drinking glass to flatten. Bake for 12-15 minutes. Half way through baking, if you notice that the crust is puffing up, remove and gently press it back down using the bottom of the drinking glass. Continue to bake until the crust is dry and set. Let cool on a wire rack as you prepare chocolate filling.
In a medium sauce pot, bring cream to a boil. Remove from heat and add in chopped chocolate. Whisk until smooth. Pour into the prepared tart shell. Smooth out chocolate ganache into an even layer. Chill in refrigerator for one hour, until set.
Roughly chop the remaining 1/3 cup of hazelnuts. Sprinkle hazelnuts and sea salt flakes on chilled tart just before serving.
*Tart can be made a day ahead of time. After tart has chilled and set, cover tightly with plastic wrap or store in an airtight container in the fridge overnight. Sprinkle hazelnuts and salt before serving.
ADAPTED FROM REAL SIMPLE DEC 2010