Put cream into mixing bowl and mix on high until soft peaks form, about 4 minute Refrigerate for 1 hour.
Whisk egg whites and sugar in a bowl while in a pan of boiling water and temp reaches 160 deg(this helps the sugar fully dissolve).
Attach bowl to mixer with the whisk attachment; beat on high speed until slightly cooled and stiff peaks form (but not dry), about 5 minutes.
Switch from whisk attachment to paddle attachment. Set on medium and add butter, several pieces at a time, mixing well after each addition (meringue will deflate slightly as butter is added). Add vanilla, hazelnut cream (you can add 1 tsp hazelnut syrup **not extract** to the "1/3 cup heavy cream", if you cannot find hazelnut cream), salt; mix until mixture comes together.
Fold in the whipped cream that has been put aside in the fridge.
Fill your favourite pastry.
This saves well, refrigerated, but will firm up. Best to let it come to room temperature when using later.
Time includes the time it takes for whipped cream to set up in the fridge.