Line the baking tray with baking paper. Preheat the oven at 190°C/375°F. Sift the flour onto the paper, set aside. Put the butter and milk 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 5 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 and milk 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 12 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 whipping cream and sugar into a mixing bowl, whip it up until medium stiff peaks. Keep in the fridge before using.
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.
Cut the cake into two same-shaped pieces.
Brush with the simple syrup, then spread the whipped cream onto it. Spread the raspberry jam onto the cream then place the other sponge cake on top. Brush with the syrup again.
Pipe the whipped cream on top and sprinkle with a little powdered sugar, then arrange fresh fruit for decoration.