Step 1Line a small shallow baking pan (such as a cake pan) with plastic, overlapping pieces if necessary, leaving extra plastic to fold back and cover the top of the pan.
Step 2To make the ganache, place the milk chocolate in a food processor. Pulse just until finely chopped. Add the Frangelico.
Step 3Heat the cream over medium heat until gently boiling.
Step 4With the food processor running, pour the hot cream in a steady stream through the feeder tube. Stop the machine as soon as the cream is thoroughly incorporated and the chocolate is smooth and shiny.
Step 5Add the hazelnut butter. Blend until thoroughly combined.
Step 6Scrape the mixture into the plastic-lined pan. Cover the top with plastic and refrigerate until slightly hardened, about 1 hour.
Step 7Heat the oven to 350 degrees. Spread the chopped hazelnuts on a baking sheet and bake, stirring occasionally, until fragrant and golden, about 5 to 7 minutes. Allow the nuts to cool.
Step 8Using a very small ice cream scoop or melon baller, scoop out equal-sized pieces of the ganache and arrange them on a parchment-lined baking sheet until all the chocolate mixture is used. (Dip the scoop in hot water between portions to make ganache easier to remove.)
Step 9Cool your hands by rinsing them under cold tap water. Dry your hands and roll the ganache portions into balls. Refrigerate for 30 minutes.
Step 10Chop the dark chocolate into pieces no larger than one-fourth inch and melt it in the top of a double boiler over low heat (the water should be at a very gentle simmer). Make sure the double boiler top is immaculately dry, as any moisture will cause the chocolate to seize. Remove the chocolate from the heat when a few pieces remain unmelted and stir to melt the rest of the chocolate. Cool slightly.
Step 11Line a baking sheet with parchment paper. Drop a few ganache balls in the melted chocolate, then remove them with a fork and drop them into the hazelnuts. Roll in hazelnuts to cover and arrange on the parchment. Repeat until all the truffles are assembled.
Step 12Refrigerate the truffles until the chocolate is set, about 10 minutes. Store in an airtight container at room temperature for up to a week.