Whisk together the melted butter, sugar, vanilla extract and salt. Stir in the flour and mix together until it resembles damp, course sand. Press the dough into the tart pan evenly on the bottom and up the sides of the tart pan. Prick the dough evenly all over with a fork and refrigerate for about 30 minutes.

When ready to bake, preheat your oven to 350 degrees. Set your tart pan onto a baking sheet/tray and bake for about 25 minutes, or until golden brown.

Bring the heavy cream, milk, sugar and salt to a low simmer over low heat. Remove from the heat and add the chocolate when you start to see little bubbles starting to form around the edges. Let sit for about 2 minutes without stirring and after 2 minutes, begin whisking from the center out, until the chocolate is smooth and shiny.

Add the beaten egg and mix well. Next, add the roasted hazelnuts and mix well again. Pour the filling into the hot crust. Lower the oven temp to 300 degrees and bake the tart for about 15-16 minutes, or until the tart has set. You know it's good to remove from the oven when you jiggle the tart pan and only the center portion jiggles. If it jiggles all the way to the outer edge of the tart, you need to bake it a few minutes longer.

Remove from the oven and sprinkle with the course sea salt, being as generous as you please. Let the tart cool for a while before attempting to remove it from the pan.