Preheat a conventional oven to 180°C (356°F). In a mixing bowl, combine the biscuit mix, eggs, and 25 grams of water. Whip on high speed for about 10 minutes until the batter becomes light and airy. Finish by mixing the batter for 1 more minute on the lowest speed to stabilize the texture. Pour the batter onto a baking sheet lined with parchment paper and spread it evenly with a spatula. Bake in the preheated oven for 10–15 minutes, or until the sponge turns golden brown and springs back to the touch.

While the cake is baking, lay a clean kitchen towel flat on the counter and generously dust it with powdered sugar. Once the sponge is baked, immediately turn it out onto the prepared towel, parchment side up. Peel off the paper carefully and, using the towel, roll the cake up tightly from one short side. Let it cool completely on a wire rack with the towel still wrapped around to help it keep its shape.

Meanwhile, prepare the Swiss cream filling. In one bowl, whisk the pastry cream mix with 250 grams of water for 2 to 3 minutes until smooth and creamy. In a separate bowl, whip the cream until it holds stiff peaks. Gently fold the prepared pastry cream into the whipped cream until well combined. Keep the mixture refrigerated until needed.

Once the sponge has cooled, gently unroll it. Spread a thin, even layer of jam or fruit filling across the surface. Then, spoon and spread the Swiss cream evenly on top of the jam. Carefully roll the cake back up without the towel, keeping it tight but gentle to avoid cracks.

If desired, garnish the cake roll with a dusting of powdered sugar, fresh fruit, or piped whipped cream before serving.