Line the baking tray with baking paper. Preheat the oven at 180°C/360°F. Sift the flour onto another piece of paper, set aside. Put the butter, milk, and vanilla extract into a small pot, warm it up to melt completely.
Put the egg and sugar in a bowl. Prepare the hot water in a pot and place the bowl over it, warm the egg until it becomes tepid.
Transfer the warmed egg into a mixing bowl, then whip it up with a whisk attachment at the highest speed for about 3 min. Then turn to the lower setting, whisk the batter for another 2 min. Lift the whisk and use it to draw a line in the batter. If the line doesn’t fade away, it's done.
Add the sifted flour to the egg mixture little by little, mix with the rubber spatula from bottom to top.
Add some cake batter into the butter, milk, and vanilla extract mixture, mix it, then add it to the cake batter, mix from bottom to top. Be careful to mix quickly and as little as possible.
Once the batter is well combined, stop mixing and spread it into the baking tray. Bake in the preheated oven for about 15 minutes. Touch the surface gently, if you feel a little resistance, it’s done. Transfer the sponge cake onto a rack immediately then place another baking paper onto the sponge cake. Let it cool at room temperature.
Put the water and sugar in a pot. Heat on medium high and stir occasionally with a whisk. Once it boils, stop heating. Let it cool.
Put the whipping cream and sugar into a mixing bowl, whip it up until medium peaks. Keep in the fridge before using.
Using a ruler, cut the sponge cake into equal width pieces. The height of the cake will vary depending on the width. Cut it according to the size you like. However, please make sure all the pieces are the same width to make your cake is evenly flat.
Brush with the simple syrup, then spread the whipped cream evenly onto the sponge cake. The amount is up to you, but make sure to keep some cream for decoration.
Place the sliced strawberries all over the sponge cake. We're gonna roll it up after this, so try not to place the strawberries so they overlap two sponge cake strips. Spread the cream over the strawberries, just to fill the gaps between them.
Beginning with a cake strip at the edge of the sponge cake, roll the cake up gently and place it onto your favorite plate. Then wrap the rest of the sponge cake around it to make a circle.
You can finish this cake by piping cream and decorating with some fruit. I frosted my cake this time. How to decorate is totally up to you, so please have fun with it!