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While the roulade is baking, prepare the filling. In a saucepan over medium heat, mix the condensed milk and milk powder until homogeneous. Then add the heavy cream and cook the mixture, stirring constantly, until it thickens slightly. It's essential to maintain a creamy, moist texture rather than allowing it to become too firm.
Once the filling reaches the right consistency, remove it from the heat and let it cool slightly while the roulade finishes baking.
Assembling the Roulade
After baking, remove the roulade from the oven while it's still warm to prevent it from drying out. Turn it over onto a clean surface and gently peel off the baking paper. Allow it to cool for just a moment before applying the filling to the surface.