Preheat oven to 350°. In a microwave or double boiler, melt chocolate.
In a large bowl, cream butter with granulated sugar until light and fluffy. Add eggs, vanilla, and melted chocolate and beat to combine. Add flour, soda, and salt. Stir in oats and hazelnuts.
Break off small pieces of dough, roll into 1 1/2-in. balls, and arrange on parchment-lined baking sheets. Bake until puffed, about 9 minutes. Transfer to a cooling rack and sift powdered sugar over cookies.