In a microwave-safe bowl, combine the sugar and cornstarch and mix well. Add the egg yolks, vanilla extract, and milk. Whisk everything together until smooth. Microwave on high for 3 minutes. After this time, remove and stir well, then microwave for another minute. Repeat until you reach the desired consistency, about 4 or 5 minutes total. For me, it took 4 minutes. Let it cool slightly, cover with plastic wrap touching the surface, and refrigerate.

Let the pastry cream cool for a few hours, or you can make it a day ahead. Preheat the oven to 350°F (180°C), top and bottom heat.

Separate the egg yolks from the whites. Beat the egg whites until stiff peaks form and set aside. Add the sugar and vanilla to the yolks and beat until the mixture is pale and creamy.

Gently fold the beaten egg whites into the yolk mixture using a spatula, then gradually fold in the sifted flour with gentle, folding motions. If needed, mix just enough to fully incorporate the flour.

Line a baking sheet with parchment paper and spread the batter evenly with a spatula. Bake for 10 minutes, then check for doneness with a toothpick. Place a clean kitchen towel on top, flip the cake over, remove the parchment, and roll the cake up with the towel. Let it cool.

In a saucepan, combine the water and sugar over medium heat. Stir until all the sugar dissolves, then let it cool.

Once the cake has cooled, about 20 minutes, unroll it. Brush with the syrup and spread the pastry cream evenly over the surface.

Roll the cake back up, cover, and refrigerate. When ready to serve, dust with powdered sugar and enjoy.