Preheat your oven to 300°F (150°C) and line two round cake pans with parchment paper. Beat the aged egg whites until frothy, gradually add granulated sugar, and whip until glossy and stiff peaks form (about 8-10 minutes).

Combine ground roasted hazelnuts, sugar, and all-purpose flour in a separate bowl and stir until well blended.

Gently fold the dry ingredients into the whipped egg whites, preserving the airy texture using a spatula.

Divide the meringue mixture between the prepared pans, smooth the tops, and bake for approximately 2 hours until dry and lightly golden.

Turn off the oven and let the layers cool inside for 12-24 hours with the door slightly ajar.

For the Charlotte cream, heat milk and sugar gently in a saucepan. Whisk egg yolks and temper them with the warm milk, return to the heat until thickened. Beat in softened butter after cooling.

Assemble the cake by placing one meringue layer on a serving plate, spreading a layer of Charlotte cream on top, then add the second meringue layer and repeat until stacked.

Cover the assembled cake with plastic wrap and refrigerate for at least 4-6 hours or overnight to allow flavors to meld.