Preparation Stir milk chocolate in medium metal bowl set over saucepan of barely simmering water (do not allow bottom of bowl to touch water) until melted and smooth. Remove chocolate from over water. Whisk in Nutella and chopped hazelnuts. Stir bittersweet chocolate in another medium metal bowl set over saucepan of barely simmering water (do not allow bottom of bowl to touch water) until chocolate melts and candy thermometer registers 110°F. (chocolate will feel warm to touch). Remove from over water. Using 1-inch-wide pastry brush, coat insides of candy cups with just enough chocolate to cover. Rewarm chocolate as necessary to maintain temperature. Place cups on baking sheet. Chill until chocolate is firm, about 15 minutes. Spoon enough hazelnut mixture into center of each chocolate cup to fill to within 1/8 inch of top (about 1 heaping teaspoonful in each cup). Refrigerate until filling sets, about 15 minutes. Rewarm remaining melted bittersweet chocolate in bowl set over barely simmering water to 110°F. Spoon enough chocolate over filling to cover and to fill cups completely. Immediately top each with whole nut. Refrigerate cups until firm, about 20 minutes. (Can be made 2 weeks ahead; refrigerate in single layer in airtight container.) Let stand 15 minutes at room temperature before serving.
Preparation Stir milk chocolate in medium metal bowl set over saucepan of barely simmering water (do not allow bottom of bowl to touch water) until melted and smooth. Remove chocolate from over water. Whisk in Nutella and chopped hazelnuts. Stir bittersweet chocolate in another medium metal bowl set over saucepan of barely simmering water (do not allow bottom of bowl to touch water) until chocolate melts and candy thermometer registers 110°F. (chocolate will feel warm to touch). Remove from over water. Using 1-inch-wide pastry brush, coat insides of candy cups with just enough chocolate to cover. Rewarm chocolate as necessary to maintain temperature. Place cups on baking sheet. Chill until chocolate is firm, about 15 minutes. Spoon enough hazelnut mixture into center of each chocolate cup to fill to within 1/8 inch of top (about 1 heaping teaspoonful in each cup). Refrigerate until filling sets, about 15 minutes. Rewarm remaining melted bittersweet chocolate in bowl set over barely simmering water to 110°F. Spoon enough chocolate over filling to cover and to fill cups completely. Immediately top each with whole nut. Refrigerate cups until firm, about 20 minutes. (Can be made 2 weeks ahead; refrigerate in single layer in airtight container.) Let stand 15 minutes at room temperature before serving.