1. In a saucepan over medium heat, combine the blueberries, water, sugar and lemon juice. Stir frequently, and bring to a low boil.
2. In a small bowl, mix the cornstarch with water.
3. Slowly stir the corn starch into the blueberries, taking care not to crush the blueberries. Simmer until the homemade blueberry sauce is thick enough to coat the back of a metal spoon, about 5 minutes.
4. Remove from heat and gently stir in vanilla.
1. Put egg yolks & 20g sugar in a bowl, mix till combine. Add oil, milk and mashed banana till well combine.
2. Fold in sifted flour & mix till combine.
3. In a clean bowl, whisk the egg whites till frothy & add tartar powder. Add remaining sugar gradually and beat till stiff.
4. Fold in 1/3 of the whites into the yolk mixture till well incorporate. Pour yolk mixture into egg whites & fold gently till well combine.
5. Pour the batter into baking pan lined with greased baking paper. Smooth the surface with spatula. Bake in the pre-heated oven 190C for 15 min.
6. Once done, Leave to cool on wire rack (about 5 minutes).
7. Invert the cake & gently remove the baking paper. Turn the cake over. With a knife cut 2 lines (at the beginning of rolling point) but the cake is still attached to each other. Spread the sauce evenly on the surface. Leave 1-2cm remain unspread.
8. Hold the cake with both hands and roll gently. When the swiss roll is rolled up, make sure the ending portion facing downward. Keep the roll in the refrigerator at least 30 minutes before cut and serve.