Very gradually, mix enough of the cream into the hazelnut paste to make it soft and similar in consistency to lightly whipped cream. Whip the remaining cream to soft peaks.
Fold the cream into the hazelnut paste together with the toasted nuts. If the cream is over-whipped, it can break when the rest of the ingredients are added. If it is not whipped enough and the filling seems too runny, just mix a little longer.