Preheat your oven to 180 °C (350 °F) conventional heat. Line a baking sheet with parchment paper.

Insert the butterfly whisk attachment into your Thermomix®.

Add the eggs and sugar to the mixing bowl. Mix for 6 min / 37 °C / speed 4.

Turn off the heat function and mix again for 6 min / speed 4.

Combine the flour, cornstarch, and baking powder, sift into the mixing bowl, and blend for 4 sec / speed 3.

Remove the butterfly whisk, scrape down the flour from the sides with a spatula, and gently stir a bit more by hand.

Pour the cake batter onto the prepared baking sheet, spread it smooth, and pop it straight into the oven. Bake for 10 minutes until light golden yellow.

While the cake bakes, lay out a clean kitchen towel.

You'll flip the baked sponge onto this towel. Simply grab the top two corners of the parchment paper and flip the cake quickly onto the towel so the parchment paper is facing up.

Dampen the parchment paper with a wet sponge or cloth, then carefully peel it off from the side. This helps the cake release effortlessly without sticking to the paper!

Place the parchment paper loosely back on top of the sponge. Now, invert the still-hot baking sheet over the cake to cover it. This traps the moisture inside, keeping the sponge soft, flexible, and crack-free when rolling. Let the cake cool completely like this.

While the sponge cools, prepare your filling.

Rinse and dry the mixing bowl and butterfly whisk.

Add the chilled heavy cream to the mixing bowl. Whip on speed 3 until desired stiffness is reached. (It took me about 2 minutes and 30 seconds, but this depends on how cold your cream and bowl are! Keep a close eye on it after the 2-minute mark so you don't accidentally make butter).

Add the low-fat quark, sifted powdered sugar, vanilla sugar, whipped cream stabilizer, lemon zest, and lemon juice. Mix for 5 sec / speed 3 until incorporated.

Spread the filling evenly over the cooled sponge cake using an offset spatula or cake server, leaving a small border free along the top edge.

Using the kitchen towel to help guide you, roll up the sponge cake roll tightly and place it seam-side down onto a flat serving platter.

To finish, dust generously with powdered sugar and decorate with fresh lemon slices.