The day before baking the cake, prepare the kyohou grape compote Recipe ID: 1214074 using 100 g of sugar, and let set in the refrigerator overnight.
Place the white chocolate in a bowl. Heat the heavy cream in a pot until just before it starts to boil, then pour over the white chocolate and mix to melt. Be careful not to let it boil!
Place the bowl in cold water to cool while stirring. When it is completely cooled, place in the refrigerator to chill overnight. Be careful not to incorporate air into the mixture!
To make the sponge cake, line a baking tray with a layer of parchment paper cut to fit the base of the tray, and on top of that, lay another layer of parchment paper, large enough to cover the edges of the tray.
Combine the cake flour and wheat starch and sift well. Separate the egg yolks from the whites and place in separate bowls. Preheat the oven to 200 degrees Celsius.
Melt the butter over a double boiler until warm.
Lightly beat the egg whites with a hand mixer. Gradually add the trehalose and half of the sugar 1/3 at a time, beating on low speed until peaks form. If using just sugar, add 2/3 of the sugar.
Finally, keep beating the eggs slowly until you can draw a circle in the batter. Be careful not to over-mix.
Add the remaining sugar to the egg yolks and beat with a hand mixer until thick and white.
Switch to a whisk and add a scoop of the egg white from Step 8 to the egg yolks from Step 9 and mix until it is smooth and drops off the whisk in clumps.
Add the remaining egg whites and incorporate in the same fashion as Step 10. So as to not deflate the foam, lift the batter between the prongs of the whisk, then let it drip back into the bowl.
Once the batter is smooth, sift the dry ingredients one more time, then incorporate it into the batter by scooping it up with the whisk, then letting it drip back into the bowl.
Switch to a rubber spatula and mix in the warm butter, scraping the bottom of the bowl. Be careful not to deflate the air bubbles in the batter.
When the butter is completely incorporated and the batter is shiny, it's ready.
Pour the batter into the baking sheet lined with the parchment paper. Smooth the surface. Be careful not to deflate the air bubbles by smoothing it too much.
Lightly tap the baking try to release any large air pockets and bake in an oven preheated to 200 degrees Celsius for 9 minutes. If you prefer a fluffier cake, bake at a higher temperature for a shorter time.
Once baked, drop the cake in the baking tray twice onto a hard surface, then remove it together with the paper. Release the paper from the sides of the cake only, then flip the cake over so that the top part of the cake is now facing down on the bottom layer of the parchment paper.
Flip it over and place in a plastic bag to cool while steaming from the residual heat. Watch out for condensation.
To prepare the filling, heat the compote liquid in a heatproof bowl in for 3 1/2-4 minutes to simmer. It'll boil, so use a rather large bowl.
Let it completely cool as-is. As it cools, it will thicken to a sauce.
Remove the top paper from the cooled sponge cake and flip onto a new piece of parchment paper and remove the other piece of parchment paper.
Use your fingers to remove the remaining browned part of the cake. It will melt in your mouth.
Cut the edge of the cake opposite the edge from where you will roll at an angle, and make shallow cuts along the cake (in the direction you will roll it).
Brush the compote liquid over the surface of the cake.
Beat the cooled cream to a creamy and yoghurt-like consistency. Add the simmered liquid from Step 20 and beat until stiff peaks form.
Spread the cream over the sponge cake from Step 24, starting thickly at the edge from which you will roll, and spreading it toward the opposite end. Leave the angled edge free of cream for the seam. Leave some cream for the topping.
Arrange a row of drained grapes on the edge from which you will roll, then cleanly and tightly roll up the cake using the parchment paper to make a firm core.
When the cake is rolled, place it on a working surface with the seam side down. Use a ruler to hold down the paper while pulling it out from under the roll.
Wrap with plastic wrap and chill in the refrigerator with the seam side down. This will help it hold its shape.
Trim off the ends and use a tea strainer to dust powdered sugar on top. Use a pastry bag to pipe the leftover cream on top to decorate.
Dip the grapes in nappage, and place them on top of the cream with mint leaves to finish.
If you don't have any wheat starch, you can substitute it with corn starch, but this may produce a drier cake.
If you use wheat starch, the cake will be fluffy and melt in your mouth. You can usually get it at large supermarkets.
The compote is so juicy.
Story Behind this Recipe
I wanted to make something using kyohou grape compote.
When combining the dry ingredients in Step 12, mix until well combined, but be sure not to over-mix or the batter will deflate. If the butter is cold, it will sink to the bottom and will be hard to mix, and will cause the cake to go flat, so be sure to properly warm it. Be careful not to deflate the batter, or the dough will become tough.