Use a serrated knife to slice; it’s easier when the cake is cold or partially thawed, but allow it to come to room temperature before serving, as the chill blunts the flavor.
Preheat oven to 350°F. Spread hazelnuts in a single layer on a baking sheet. Toast for 4 to 5 minutes or until golden brown. If hazelnuts have skins, place in batches in a kitchen towel and rub vigorously to remove skins. Cool to 225°F. Line 1 or 2 baking sheets with parchment paper an