Whisk the egg whites with 2 Tbsp of sugar until thick and pale and very foamy. Mix the rest of the sugar with flour, potato starch and baking powder, then sift into the egg mixture and fold in gently.
Fold in the cooled melted butter.
Pour the batter into a buttered bundt cake pan and bake in a pre-heated 350F oven for 30-40 minutes. Test for doneness with a wooden toothpick.
Cool slightly before removing cake from pan onto a serving plate.