Assembly by The Spiced Life, individual mousses adapted from Alice Medrich. Both mousse layers need to chill for 1 or more hours, so you can, as I did, split this up into 2 days.
Ingredients
For dense minty dark chocolate layer
8 oz bittersweet chocolate, in the 70% range, chopped
8 T (1 stick) unsalted butter, cut into chunks
4 large eggs
⅓ cup sugar
⅓ cup water
a few drops of peppermint oil
For peppermint white chocolate mousse:
9 oz white chocolate, finely chopped (use a good quality chocolate; it should have cocoa butter in it)
6 T water
1½ cups heavy cream
drops of peppermint oil, to taste (see instructions)
Instructions
First make the dense bittersweet chocolate layer: Melt the chocolate and butter either in a double boiler or in the microwave on 50% power. Add the few drops of peppermint oil. The chocolate should taste pleasantly, lightly minty. Whisk smooth and set aside.
Fill a large skillet partially with water and bring it to a simmer. Using a heat proof bowl, place the eggs and the sugar in the bowl. Turn the heat down on the water so that it is barely simmering and place the heat proof bowl in it. Stir until well-blended and then add the water. Keep stirring constantly to keep the eggs from cooking. I did this 2 handed with a thermometer in the bowl, but Medrich notes you may need to continuously remove the bowl to check the temperature (in which case clean the thermometer inbetween tests); either way, keep stirring until the temperature reaches 160 F. Remove the bowl from the skillet and use an electric mixer to whip the eggs on high speed for 3-4 minutes, or until they have a texture like lightly whipped cream.
Fold ¼ of the egg mixture into the chocolate. Then fold in half of the remaining egg mixture. Then fold the rest of the eggs into the chocolate mixture. Fold until no streaks remain. Working quickly (because the chocolate mousse can begin to stiffen), divide the mousse between your glasses. Chill for at least one hour (can be done the night before).
Then make the white chocolate mousse: Place the chocolate and water in a heat proof bowl. Either melt over a double boiler or in the microwave at 30% power for 3 minutes. Add the peppermint oil--the taste should be quite minty, unlike the recipe above. You are going to add a lot of cream to this, which will dilute it, plus my family likes a strong peppermint flavor. Stir until completely melted and smooth and set aside to cool. The chocolate must cool to about 85 F (Medrich notes a small dab on the lip should feel cool; I used a thermometer)--do not let it cool lower than 85 F as the mousse will end up grainy. When the mousse is getting close to 85 F, it is time to fix the cream.
Whip the cream in a medium bowl "only until it is thickened and barely beginning to hold a shape--when you tilt the bowl it should flow to one side, fluffy but still pourable..."--I have quoted Medrich here as she emphasizes that you will think the cream is not whipped enough but it is and she is right. It should barely be whipped.
Scrape the cream into the 85 F bowl of chocolate and quickly but gently fold the 2 together until they are completely blended. The mousse should seem too soft--it still needs to chill. Divide it up between your serving glasses and chill until set. This will be at least an hour for individual servings, more for a large bowl. I gave mine 2-3 hours to be certain.
Just a quick note that while I am by no means an expert on chocolate curls, I have found over time that a serrated peeler works well (lacking a proper chocolate curler) and it helps immensely if the chocolate is quite warm. I heat mine in the microwave on 2% power until it is just barely starting to get melty at the edges. My kids thought the chocolate curls were the best part of the whole thing!
Recipe by The Spiced Life at https://www.thespicedlife.com/double-chocolate-peppermint-mousse-parfait/