Preparation: Combine all the ○ingredients and sift the flour. Line the bottom and sides of the cake pan with baking parchment.
Fill a double broiler with water heated to about 60°C. Melt the butter, and place over a separate double broiler so that it doesn't cool and harden.
Place a bowl over the double broiler, so that the bottom is submerged in the water. Add the eggs and sugar, and beat the mixture with a whisk while heating until warm to the touch (do not overheat.)
Remove the bowl from the double broiler. Using an electric mixer set to high, beat until stiff peaks form and the whisk leaves a trail in the mix. This should take about 5 minutes if you are making a 15 cm cake.
Reduce the whisk's speed to medium, and beat for a further 3-4 minutes until the batter becomes foamy. Reduce speed to low, and whisk the batter for 1-2 minutes more until shiny and smooth.
※ You want to beat until the batter is slightly gloopy and shiny - it should feel heavy when you run the whisk through it. When dropped from the whisk, the batter should sit on the surface without sinking back into the mixture (as photographed).
Quickly fold in the the sifted flour with a spatula, while rotating the bowl, so that it is fully incorporated.
Even after the flour is no longer visible, continue folding for a bit longer until the mixture becomes glossy. (You will fold the batter about 40 times in total)
Melt the ○ingredients, either in a microwave or over a double broiler. You want to bring the mixture to about 50-60℃. Take a small amount of the Step 8 mixture and mix it into the butter mixture.
Pour the Step 9 butter mixture evenly onto the surface of the Step 8 batter, and using a rubber spatula mix just enough to incorporate.
Pour the batter into the cake pan. Rap the bottom of the pan against your work surface, to release any trapped air bubbles.
Bake for 20 minutes in an oven preheated to 180°C. Reduce the temperature to 170℃ and bake for another 8-10 minutes. If an inserted skewer comes out clean, it's finished baking.
When the cake is done, drop the pan onto your work surface from a small height. (This prevents the cake from shrinking.)
Turn the cake upside-down on a wire rack and remove from the pan straight away. Leave to cool slightly, then turn right-way-up. Cover with a cloth so that it doesn't dry out, and leave to cool completely.
Excluding the browned top, the cake should be tall enough to cut into three 1.8 cm slices. It's up to you whether or not you cut off the browned surfaces.
Sponge cakes usually use butter for fat, but using half butter and half vegetable oil allows the cake to rise more. In total, it should be about 6-6.3 cm high.
※When using a 18 cm cake pan, adjust the whisking time to 8 minutes on high speed, 5 minutes on medium speed, and 2-3 minutes on low speed. The baking time will also need to be increased by 2-3 minutes.
This sponge is so moist, fluffy, and light, you'll hardly feel any resistance when cutting into it with a knife.
Story Behind this Recipe
I wanted to come up with a no-fail recipe for classic sponge cake.
Don't line the sides of the pan with parchment paper, as the cake sinks away from the sides during cooking. Adjust the length of time you mix for according to your electric whisk. I suggest using cake flour with a low protein content.