Filling: Whisk sugar, corn starch, lemon juice, eggs and egg yolk in heavy saucepan over medium heat. Cook, stirring constantly for 8 to 10 minutes, or until thick. Stir in sour cream and butter. Chill 15 to 20 minutes.
Sponge Flan: Preheat oven to 350°F (180°C). Butter a 9-inch (23cm) springform pan and line with parchment paper. Whisk eggs, sugar and salt in a large metal bowl set over simmering, water until very light and thickened, about 8 minutes. Mix the flour with the corn starch, baking powder and salt. Add to egg mixture and fold in gently. Fold in butter and vanilla. Scrape into the prepared pan and smooth the top.
Bake for 18 to 20 minutes or until cake springs back when touched. Cool on a rack. Remove the pan ring. Spread the filling over the cake.
Meringue: Preheat oven to 400°F (200°C). Beat egg whites until soft peaks form. Gradually beat in sugar until stiff peaks form. Mound the meringue evenly over the filling. Bake for 5 to 7 minutes or until lightly browned.