The first time I had a Mervielleaux I as in college. It was thick layers of meringue discs and rich chocolate buttercream in a cute dome. I hadn't thought about it again until I saw an Instagram post about it a few weeks ago and decided to google.
They are both simple, delicious, and frustrating. The simple part was it was just meringue layered with chocolate Chantilly and iced in vanilla Chantilly. That also explains why it was delicious. However Chantilly is liquid and the meringue dissolves pretty fast. They need to be assembled and eating right away. I have no idea how Aux Merveilleaux de Fred stabilizes theirs but mine didn't last long.
Meringue is typically cooked at a low temperature to keep them from browning. I like them to brown slightly, it adds a toasted marshmallow taste to the finished product.
6 egg whites
1 1/2 c. powdered sugar
1 tsp. vanilla extract
1 tsp. white or cider vinegar
6 c. heavy cream
1 - 2 c. sugar, divided
1/4 c. cocoa powder
pinch xanthan gum
12 oz. shaved chocolate
Heat the oven to 275 and line a pan with parchment paper.
Beat the egg whites until foamy. Add the salt and keep beating. Then slowly add in the sugar in a thin stream while still beating. Beat until stiff peaks form. Add in the vanilla and vinegar and fold to mix in.
Fit a pastry bag with a #12 pastry tip. Pipe the meringue in to 2 inch disks at least an inch apart or use a spoon to shape. The discs should be fairly thick. Smooth the top flat.
Place the pan in the oven and bake for 1 hour. Turn the oven off and leave the door closed until the oven in completely cool.
When the meringues are completely cool combine 2 1/2 c. of the heavy cream, the cocoa, and 1/2 c. sugar (or more to taste). Beat until stiff peaks form. Place in to a piping bag with a small start tip.
Layer a meringue disk, pipe the top with the chocolate cream. Place another meringue dish on top and them more cream. Top with a third meringue disk and set aside. Once all the meringues are used up there should be a little of the chocolate remaining. Store in the fridge until ready to use again.
Beat the remaining cream, 1/2 c. sugar, and xanthan gum until stiff peak form. This too gets placed in a pastry bag. Pipe the whipped cream on to the sides of the meringue towers. Rollin the shaved chocolate. Pipe more whipped cream on the top and finish with a chocolate swirl to use up the remaining chocolate cream.