In a large metal bowl, beat softened butter, sugar, and chocolate together.
Beat in eggs and vanilla. Using a wooden spoon, gradually stir in flour mixture until well combined.
Chop roasted hazelnuts and fold into mixture. Roll into 1 1/2 inch (4 cm) balls leaving at least 1 inch (2.5 cm) Â between cookies. Flatten with the bottom of a jar or glass cup.
Bake for 13 to 15 minutes.  Let cool on a wire rack.