Hazelnut Filling

Put cream into the bowl of an electric mixer fitted with the whisk attachment; beat on medium-high speed until soft peaks form, about 4 minutes. Transfer to a medium bowl. Refrigerate 1 hour.

Whisk egg whites and sugar in the clean bowl of mixer set over a pan of simmering water until sugar has dissolved and mixture registers 160 degrees, 2 to 3 minutes.

Attach bowl to mixer fitted with the clean whisk attachment; beat on high speed until slightly cooled and stiff (but not dry) peaks form, about 5 minutes.

Fit mixer with paddle attachment. With mixer on medium-low speed, add butter, several pieces at a time, mixing well after each addition (meringue will deflate slightly as butter is added). Add vanilla, hazelnut cream, and salt; mix until mixture comes together, 3 to 5 minutes. Fold in whipped cream with a rubber spatula. Use immediately.