Line a 30 cm square baking sheet with parchment paper. Sift the flour twice. Separate the egg yolks from the whites. Put the egg whites into a large bowl.
Beat the white eggs until frothy and no longer gel-like. Combine 30 g of white castor sugar and 20 g of ◇ trehalose. Add to the egg whites in 2 to 3 batches and continue to beat.
For this recipe, a soft meringue is preferable, so beat the egg whites until glossy and has soft peaks that fall over. Stop beating just before stiff peaks form.
Put the egg yolks and 30 g of the ◆white castor sugar in a separate bowl and suspend over a pan of hot water (remove from hot water when the mixture is warm to the touch). Whisk until thick and heavy.
Put the butter and milk in a heatproof dish and microwave for about 20 to 30 seconds on high to melt the butter. Preheat the oven to 180°C.
Add the warmed Step 5 mixture and vanilla extract to the Step 4 and stir well. Add the flour and fold in with a hand whisk and rubber spatula without mixing vigorously.
Scoop up the meringue with a hand whisk and add to the mixture from Step 6. Using a plastic spatula, stir in the mixture stuck on the sides of the bowl.
Scoop 2 portions of meringue with a hand whisk and stir in until evenly combined. Pour the Step 7 mixture into the meringue and stir quickly until evenly combined.
Pour the batter into the baking sheet. Level the surface gently and drop the baking sheet from 10 cm height twice to get rid of big air bubbles.
Bake in a preheated 180°C oven on the the bottom rack for about 13 to 15 minutes (adjust the baking time to your oven). After baking place the sponge cake on a cooling rack to cool slightly.
After it has cooled slightly for about 5 minutes, put the sponge cake in a large plastic bag to prevent drying, and leave to cool completely.
Meanwhile, mix the double cream and castor sugar and whip until soft peaks form. Add condensed milk and continue to whip until stiff. Stiff whipped cream is easier to roll the cake later.
When the sponge cake has cooled completely, peel off the parchment paper. Decide which side will be on on the outside, and slice off an approximately 3 cm wide diagonal piece off the side for the end of the roll.
Here the top is the start of the roll and the bottom is the end. Spread the cream over the sponge except for the diagonally sliced off part. If you use all the cream, it will be quite a lot.
It's done. I rolled with the side on the parchment paper outside. If you a beginner, this is easier.
【Variation】Instead of butter, I used canola oil in the sponge cake batter, and filled with caramel cream. Recipe ID: 854416
This cream filling is Kyoho grape-flavoured. I mixed 200 g of double cream, 15 g of sugar + 50 g of "uzukai's" grape jam (Recipe ID: 908844).
Story Behind this Recipe
This sponge cake holds its shape even though it uses less flour than usual, and has a melt in your mouth texture. I reduced the condensed milk in the original recipe to make it less sweet.
Beat the egg whites until it becomes a rather soft and loose meringue. If you have a stiff meringue, it will be difficult to stir into the egg yolk mixture, and you may lose the air bubbles. When mixing the meringue with the egg yolk mixture, fold in gently and quickly until evenly combined. Do not mix vigorously.