Make the Sponge Cake: Preheat oven to 350°F (180°C). Grease and flour an 8-inch (20cm) round cake pan. In a large bowl, beat the eggs and sugar together until light and fluffy. Sift in the flour, cornstarch, and baking powder, then gently fold to combine. Pour the batter into the prepared pan and bake for 18-20 minutes, until a toothpick inserted in the center comes out clean. Allow to cool completely.
Make the Mango Mousse: In a large bowl, beat the heavy cream and sugar until stiff peaks form. In a separate bowl, sprinkle the gelatin over the cold water and let sit for 5 minutes to bloom. Microwave for 10-15 seconds until liquid. Fold the melted gelatin and diced mangoes into the whipped cream. Pour the mousse over the cooled sponge cake and refrigerate for at least 4 hours or overnight.
Make the Mango Glaze: In a small bowl, sprinkle the gelatin over the cold water and let sit for 5 minutes to bloom. Microwave for 10-15 seconds until liquid. Stir in the mango puree. Pour the glaze over the chilled mousse cake and refrigerate for at least 2 hours to set.
Assembly: Use a knife to loosen the cake from the pan and remove the springform ring. Brush the top and sides of the cake with mango juice. Decorate with fresh mango slices.
Refrigerate the cake for at least 2 more hours before serving to allow the glaze to fully set. Enjoy!