Preheat oven to 350 degrees F. and line eight 1/2-cup muffin tins with paper liners.
In a double boiler or a metal bowl set over a saucepan of barely simmering water whisk together eggs and sugar until sguar is dissolved, 1 to 2 minutes. Remove top of double boiler or bowl from heat and with a hand-held mixer beat mixture at moderately high speed until doubled in volume, about 3 minutes.
Sift flour, cornstarch, and a pinch of salt over egg mixtrue and fold in gently but thoroughly. Drizzle butter over batter and fold in gently but thoroughly.
Divide batter among muffin tins and bake in middle of oven for 12 to 15 minutes, or until pale golden brown. Cool spongecakes in tins on a rack 10 minutes. Remove spongecakes from tins and cool completely.
In a bowl combine the strawberries, mango, lemon juice, 3 tablespoons of sugar and mint and toss to combine. Let fruit stand for 20 minutes, taste and add more sugar if necessary.
Peel off paper liners from spongecakes and halve horizontally. Serve halved spongecakes with fruit and juices, garnishing with sour cream.
Adapted from a recipe by Gourmet Magazine