Pistachio Cake With White Chocolate Buttercream

• 6 egg whites
• 1 chocolate buttercream (white)
• 400 gr granulated sugar
• 240 ml buttermilk
• 210 gr all purpose flour
• 200 gr pistachio nuts (chopped toasted)
• 170 gr butter
• 140 gr pistachio nuts (chopped toasted)
• 18 gr baking powder
• 4 gr vanilla