Filling

Place 4 egg whites in a clean, dry bowl and beat with an electric mixer on medium-high speed until soft peaks form, about 3-4 minutes.

Gradually add 2 cups sugar while continuing to beat, increasing mixer speed to high, until stiff peaks form and the mixture is glossy, about 5-6 minutes.

Fold in 4 cups ground walnuts and 1/2 teaspoon vanilla with a rubber spatula using gentle, sweeping motions until just combined.

Use the filling immediately while it has a light, fluffy texture, or cover and refrigerate for up to 2 hours before use.