Preheat oven to 180C or 350F
Sift the flour and baking powder together at least five times. This will yield a light and fluffy cake. Set aside
Beat the eggs, sugar, and vanilla until light and fluffy. It should take around 5-10 minutes
Add the sifted flour and baking powder to the beaten eggs and fold with a spatula until just combined.
Line a 17 x 11-inch cookie pan with baking paper, and grease well with oil.
Pour the cake batter into prepared pan and spread evenly. Gently tap the pan on a bench surface to knock out some of the air bubbles
Bake sponge in the oven for 8-10 minutes
Allow the cake to cool slightly, but when it is still warm, sprinkle the corn-starch over the top and begin to roll length ways, using the baking paper to help you. Allow the sponge to cool completely in its rolled-up shape.
When the sponge has cooled down, gently unroll, and spread the strawberry jam onto it, followed by the whipped cream. Gently start re rolling the sponge with the filling in it.
Slice, and sprinkle with icing sugar to serve.