1/3 cup ground raw hazelnuts (see below)
1 1/2 cups flour
1 cup firmly packed light-brown sugar
3/4 cup butter, softened
16 ounces semisweet chocolate chips
1 cup whipping cream
5 tablespoons butter, cut into about 20 pieces
3 tablespoons melted butter
2 tablespoons unsweetened cocoa powder
3 tablespoons milk or half-and-half
1/4 teaspoon vanilla
1 1/4 cups sifted confectioner's sugar
About 1/3 cup darkly roasted and coarsely chopped hazelnuts
To grind raw hazelnuts, place 1/2 cup of raw (unroasted) hazelnut kernels in a food processor and grind, using pulse button, until nuts are ground into a fine, flourlike consistency.
Preheat oven to 350 F.
In a food processor, or bowl using an electric mixer, combine the flour, ground hazelnuts, brown sugar and 3/4 cup butter. Blend just until crumbly and beginning to hold together. Scrape mixture from bowl or food processor into an ungreased, 10-inch, springform pan. Press mixture firmly and evenly into bottom of pan. Bake until crust turns golden and appears cooked (it will still be very soft, however), about 30 minutes. Remove from oven and set aside to cool.
Once crust has cooled, prepare chocolate-truffle filling. Slowly melt the chocolate chips, without stirring, in a double-boiler arrangement. This will take about 20 minutes.
When chocolate has melted, place the cream in a small, heavy-bottomed saucepan over medium-high heat and warm just until steaming and small bubbles have formed around edges (if it actually boils, it's not a tragedy, but remove it from burner immediately).
Remove from heat and immediately pour cream over melted chocolate chips. Don't be alarmed when chocolate seizes up. Just keep stirring gently with a flat, wire whisk or a wooden spoon until chocolate relaxes, softens up and ultimately blends into warm cream. Keep vessel positioned over hot water because this will help keep temperature at melting point. Once chocolate is smooth and creamy, stir in the butter, 2 or 3 pieces at a time, and continue stirring until each piece of butter is melted and mixture is well-blended and smooth. When all butter has been added, there may be a few little chunks of chocolate that don't completely melt, but that's OK.
Lift vessel of truffle sauce away from hot water and wipe its outer sides with a towel so no water will drop into sauce when you tip pot to pour onto crust. Pour sauce onto surface of cooled crust, scraping out every last drop with a rubber spatula. Smooth it out evenly, then set aside to cool for about 30 minutes. Place pan in refrigerator to cool completely and set firmly.
Once truffle sauce has set firmly, prepare chocolate glaze. In a small bowl, whisk together the melted butter and cocoa powder. Add the milk and vanilla extract and continue whisking. Finally, whisk in the confectioner's sugar. If mixture seems too thick (it should be thin enough to drizzle from a spoon), whisk in a little more milk or half-and-half.
Spread chocolate glaze over surface of tart, spreading it evenly out to edges. Sprinkle surface of tart (in a random pattern) with the roasted hazelnuts.
Return tart to refrigerator to set glaze. Tart may be served chilled or at room temperature. It's up to you. It will be softer if stored at room temperature, but it isn't a food safety issue.
Makes 12 to 16 servings.