Heat the oven to 300 degrees F. In the top of a double boiler set over hot water melt chocolate and let it cool. In a bowl beat butter, with confectioners' sugar until the mixture is fluffy.
Stir in the chocolate and beat until creamy and smooth. Stir in filberts, and form the mix into a ball. Wrap in waxed paper and chill for 30 minutes.
Pinch off 1 teaspoon of the paste at a time and roll it between the palms of your hands like a ball. Arrange balls on baking sheets lined with parchment paper and bake until they've lost their glossiness, about 10 minutes. Let the balls cool on the baking sheets, then store in an airtight container.
The Yankee Cook found these delicate cookies to be extremely sweet, right between pastry and candy. For a more intensely chocolate experience, use unsweetened (baking) chocolate, and to get the strongest hazelnut complement, toast and skin the nuts before grinding.