Prepare the Filling

• Remove the butter from the refrigerator one hour in advance to soften it, making it easier to work with.
• For convenience, loosely line a loaf pan with plastic wrap. This will serve as support for the cookies and will allow us to wrap the cake before adding the final chocolate coating.
• In a saucepan, melt the softened butter, icing sugar, and egg yolks. Stir well with a whisk until obtaining a homogeneous cream. It is better to use an electric mixer to ensure there are no lumps.
• In another saucepan, melt the chocolate using a double boiler. It is important to ensure that no water drips into the chocolate to prevent it from seizing.
• Add the melted chocolate to the bowl with the egg yolks and stir well until a homogeneous cream is achieved. Using an electric mixer helps incorporate it better.
• In a shallow dish, we place milk to soak the cookies.
• We take a Núria cookie and briefly dip it in the milk. Just a moment, so it doesn't get too soft.
• We stand the cookie upright in the mold and spread the side with chocolate cream using the spatula.
• We take another cookie, lightly dip it in the milk, and attach it to the first one, repeating the process.
• We continue attaching cookies side by side with chocolate cream until we achieve a roll of the desired size.
• Once we have the desired size, wrap the roll with plastic wrap, ensuring it is tightly secured. Twist the ends of the plastic wrap, like a candy wrapper, to tighten the roll and place it in the refrigerator until the next day (or about 3 hours to cool) to prevent it from breaking when working with it.
• The next day (or after a few hours), heat the 35% liquid cream and bring it to a boil.
• When it starts to boil, turn off the heat and pour in the chocolate to coat the roll. Stir until the chocolate is fully incorporated.
• Immediately after, add the butter and stir well until it is completely dissolved.
• While waiting for the mixture's temperature to drop, continuously stir to prevent a layer from forming on the surface.
• Remove the roll from the refrigerator, unwrap it, and place it on a wire rack (it can be the oven rack) with a container underneath to catch any dripping chocolate.
• When the temperature drops to 32°C, pour the chocolate over the roll and let the excess chocolate drain off through the rack.
• Decorate to taste by sprinkling with chocolate sprinkles, M&Ms, or dusting with powdered sugar on top.
• Without removing it from the rack, leave it in the refrigerator for a while so that the coating sets and becomes firm.
• Cut it diagonally to clearly show the layers of biscuit and chocolate.