Preheat oven to 325 F. In a small bowl, combine flour with 2 Tbsp sugar. With an electric mixer on high, beat egg whites, vanilla, salt & cream of tartar until soft peaks form. Add 1 tsp lime zest & 1 tsp lime juice. Gradually add remaining 4 Tbsp sugar. Beat on high until mixture is fully incorporated, glossy & stiff peaks are formed. Fold in by hand flour/sugar mixture in thirds until fully combined, keeping batter as voluminous as possible.