I used a 30cm x 22cm (outside dimensions) sheet cake pan. You can also use a 26cm x 26cm pan.
If the pan has a nonstick coating, lightly grease it with butter and refrigerate until you need it. If the pan is not non-stick, line it with a sheet of parchment paper.
Cut 150 g of hulled strawberries into quarters, and put them into a small enamelled pan.
Sprinkle the strawberries evenly with granulated sugar, mix lightly and leave to macerate for 10 to 15 minutes.
As time passes, the strawberries will exude moisture as shown here. Start simmering over medium heat.
When it starts to bubble, turn the heat down to low. Some foam (scum) will rise to the surface, but you don't need to remove it.
After simmering the strawberries for about 3 minutes, add the lemon juice. Just a small amount, about 1/2 of a small teaspoon, is fine.
After simmering for about 5 minutes, you should be able to clear the bottom of the pan when you drag a rubber spatula over it as shown here. When it reaches this stage it's done.
Transfer the strawberry mixture to a container, and leave to cool.
Make the batter. Put the eggs and sugar in a bowl and beat at high speed with a handheld mixer for 3 minutes, while holding the bowl over a pan of 60°C or so hot water.
After 3 minutes, take the bowl off the hot water, and beat at high speed for 4 minutes while slowly turning the mixer in a circle in the batter. The batter will become foamy and light colored.
This is the batter when it's been beaten to the soft peak stage. It will be like soft fluffy marshmallow whip.
Combine the cake flour and cornstarch, and sift 3 times. Add this in 4 batches to the batter, mixing between additions. (Use the handheld mixer switched off to mix.)
When the flours are mixed in, beat on low speed for 30 seconds. We are now done with the handheld mixer. From here on we'll use a rubber spatula.
Swirl the milk on the surface of the batter, add half the cooked strawberry puree and mix in quickly, repeatedly scooping the batter up from the bottom.
When the puree is mixed evenly into the batter as shown in the step 15 photo, the batter will have less volume than before, but it's not a problem so don't worry!
Pour the batter into the prepared cake pan, and tap the bottom of the pan 2 to 3 times on your work surface to remove any big air bubbles. (Don't overdo this!)
Bake for 12 minutes in a preheated 180°C oven. If a skewer stuck in the middle comes out clean the sponge is done. Don't overbake.
Take the pan out of the oven, and immediately drop it with a bang on your work surface to push out extra steam.
Cover tightly with aluminium foil, and flip over the pan with the foil side down onto a cooling rack. Take the pan off the sponge.
The sponge will look like this.
Cover with a sheet of kitchen parchment paper to prevent the surface from drying out, and leave to cool. Make the sheet of kitchen parchment paper large since it will be used later when rolling up the cake.
While the sponge is cooling, make the cream filling.
Put the heavy cream in a bowl and whip until creamy. Add the condensed milk and the rest of the strawberry puree, and whip the cream until peaks form.
When the sponge has cooled down completely, flip it over again (the side with the strawberry bits will be on the bottom now) and take off the foil. Don't leave the sponge to cool for too long or it may crack.
Slice about 2 cm of the edge that will be at the end of the roll diagonally. Cut the fresh strawberries to be rolled into the cake into quarters.
Score the sponge in about 2cm intervals. The scores should be about 2mm deep. If you score the roll it will be easier to roll.
Spread the cream filling on the sponge as shown, and put on the strawberries. Leave 4cm of the edge that will be at the end of the roll (the edge that was cut) clear of cream.
Roll it up from the near side. Wrap up the rolled cake with the end side on the bottom in the kitchen parchment paper, and wrap again in plastic film and leave to rest in the refrigeratorfor 1 hour to finish.
A strawberry filled roll cake. It's a sweet and sour cake packed with berries, limited to the strawberry season.
※ If you cut the cake with a warmed knife, and wipe the blade clean of the cream filling after each cut, you can slice the cake very neatly.
Story Behind this Recipe
I had a lot of strawberries I'd bought cheaply, so this is a recipe I made to have as a snack. I like the moist and fluffy sponge that you can that you can just easily mix in one bowl.
Make sure the bain marie (hot water bath) doesn't get too hot. If you beat the batter for about 3 minutes over a 60°C hot water bath, it will just about warm up to body temperature. If you don't have a thermometer, the batter should be just hot enough that if you stick the tip of a finger in it feels a bit hot. Make sure not to burn yourself!