Preheat your oven to a very high heat of 450 degrees F. Make sure you rack is in the middle of the oven. Grease and line with parchment paper a large baking tray with a one inch rim. A tray about 12 x 8 would work well, or the closest you have to it would be just fine. When you have lined it with parchment paper, grease this as well, not forgetting the sides. Important or the cake will be difficult to get off the paper when baked.
In a medium bowl, sift together the flour and the cornstarch. Set aside
I always separate the eggs when they are cold as it is much easier to do this way. So take two the of eggs and separate them. In a medium sized bowl you can put the whites and then in your large mixing bowl add the 2 yolks. Set aside the egg whites. In the large mixing bowl that already has the 2 yolks you will now add the additional two eggs. Now add your sugar to the eggs and mix on a high speed until very light and about tripled in size. This usually takes about 5 minutes. Once you have done this, mix in the vanilla extract for about 30 seconds. Next, sift in the flour and gently stir it in until it is combined. You don't want to do this too long or you will deflate the batter.
Remember those egg whites you put aside, well it is time to whisk those up. Mix on a high speed until they are foamy. This usually happens straight away. Then add your dash of vinegar and continue mixing on high speed until soft peaks form. Next, add the sugar and continue mixing. This will probably only take about a minute more. Stiff peaks should form and it should have a glossy appearance.
The next thing you have to do is fold the egg whites into the cake batter. I always start with folding in a small amount first and gently stirring it around to combine it. Once I have done this, I add the rest and gently stir it in to combine it.
Pour the batter onto your prepared baking tray and make sure it is evenly spread out. Bake for 5-7 minutes. It should be risen and when touched it will spring back slightly. Remove from oven and immediately invert it onto a damp tea towel that is laid out over a baking rack. Remove the baking tray and gently peel back the parchment paper. You may notice that around the edges it sticks a bit, so just gently pull until it comes away. Once you have removed the parchment paper you are going to roll the cake up in the tea towel. You are going to roll length ways, but this I mean you are looking for a 8 inch cake not a 12 inch cake. Make sense? Once your cake is rolled up, leave it to completely cool.
While the cake is cooling, whisk the whipping cream together with the sugar and refrigerate.
Once the cake has cooled, unravel it from the tea towel. Evenly spread over the whipped cream and sprinkle over the blueberries. Roll it back together.
On a clean plate, spread out powered sugar. Gently lift the cake and roll it in the powered sugar making sure to coat the entire cake. I then usually let it sit in the fridge for an hour or so before cutting. That way, it stays together and holds the shape.
Okay, after looking back at these instructions, it looks like a lot of steps. But once you have read it over and done it once or twice, it is a piece of cake!