First, prepare the sugar syrup by mixing water and sugar in a saucepan. Place the saucepan over medium heat and bring to boil.
Let the syrup simmer for about 15 minutes. Let cool.
To make the genoise, whisk the egg whites in the bowl of a stand mixer until stiff peaks form.
In another bowl, whisk the egg yolks, sugar and vanilla until it thickens and pale.
Add the sifted flour, cornstarch and baking powder.
Mix just to combine.
Gently fold in the egg whites into the egg yolk mixture. Make sure to not deflate the batter.
Pour the batter in a rectangular pan (I have used a 11 x 17 inch pan) lined with parchment paper.
Spread the batter evenly into a thin layer (less than an inch thick).
Bake in a 400°F preheated oven for about 8 to 10 minutes.
Prepare a large sheet of parchment paper on your counter.
Once the genoise is baked, reverse it on the prepared parchment paper.
Gently take off the top parchment paper.
Brush the genoise with sugar syrup all over its surface (you may not need to use all of it).
Spread jam evenly on the genoise (or spong cake).
Gently roll the genoise into a log.
Place the log in aluminum paper, roll it in that paper to keep its shape.
Refrigerate for at least 30 minutes.
Sift the powdered sugar on top.
Slice and enjoy.