prepare the mousse:
Put 1/4 cup of milk and vanilla in a glass measuring cup. Sprinkle gelatin over milk and let stand 5 minutes. Place the measuring cup in a small pot with enough water to come halfway up the side of the pot. Heat the water and stir the gelatin mixture for 2-3 minutes until the gelatin is dissolved and clear. turn off the heat; leave the cup in the warm water until needed.
Place the chopped white chocolate in a large bowl. Heat the remaining milk in a small saucepan. Pour the hot milk over the chopped chocolate and whisk until the chocolate is melted and smooth. stir in the butter. Whisk in the gelatin mixture. Divide mixture in half. Whisk nougat powder into one half of the chocolate mixture. Allow each half to cool completely.
In a clean bowl, whip heavy cream until soft peaks form--do not overbeat. Using a large spatula, fold half the whipped cream into each half of the white chocolate mixture. The consistency will be soft. Place a piece of plastic wrap directly on the surface of each bowl of mousse and refrigerate 30 minutes.