In medium saucepan over low heat, melt together butter, unsweetened chocolate, nutella and erythritol, stirring until smooth. Add cocoa powder and stir until fully combined. Off heat, stir in hazelnut or vanilla extract and stevia.
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Chill mixture until firm but still scoopable, about 2 to 3 hours. Scoop into 1 inch balls, rolling between palms to form. Roll balls in chopped hazelnuts.