Line your loaf pan with plastic wrap, allowing the wrap to hang over the sides. This will help you remove the terrine later.
In a double boiler or microwave, melt the semi-sweet chocolate until smooth. Set aside to cool slightly. Repeat the process with the milk chocolate and then the white chocolate, ensuring each type is melted separately.
In a large mixing bowl, whip 2 cups of the heavy cream until soft peaks form. Be careful not to overwhip, as you want it to remain smooth and creamy.
Divide the whipped cream into three equal portions. Fold the first portion into the melted semi-sweet chocolate until fully incorporated. Repeat with the milk chocolate and then the white chocolate, each time being gentle to maintain the airy texture of the cream.
In a small bowl, mix the seeds from the vanilla bean with the remaining cup of heavy cream until well combined. Fold this mixture into the white chocolate layer.
Begin layering the chocolate mixtures in the prepared loaf pan. Start with the semi-sweet chocolate, followed by the milk chocolate, and finally the white chocolate. Use a spatula to smooth out each layer and ensure even distribution.
Cover the top of the terrine with the overhanging plastic wrap and refrigerate for at least 6 hours, or overnight. This allows the flavors to meld and the terrine to set properly.
Once set, carefully lift the terrine from the pan using the plastic wrap. Slice using a sharp knife dipped in hot water for clean cuts. Serve with chocolate fudge sauce or raspberry sauce and fresh raspberries for a pop of color and flavor.
