Chocolate meringue:
Place a rack in the center of the oven and preheat to 200°F (95°C).
Line two baking sheets with Silpat or parchment paper. Reserve.

Put the egg whites in a mixer bowl and whip until soft peaks form. Increase the speed and gradually add the sugar 1 tablespoon at a time until stiff peaks form. (The egg whites should be stiff but moist.)

Sift the powdered sugar and cocoa powder over the egg whites and gently fold with a spatula until homogeneous.

Fill a piping bag with the meringue.
Shape 3 circles of 7″ (18 cm) on the prepared baking sheets.

Bake for 2 hours until the meringues are dry and crispy. Let cool and store in an airtight container for up to 3 days.

Chocolate mascarpone mousse:
Put ¾ cup (125 ml) of 35% cream and the lemon zest in a saucepan over medium-high heat.
Once hot, add the chocolate and whip until melted and smooth.
Transfer the mixture to a bowl and let it rest at room temperature until cooled.

Place the mascarpone, remaining cream, and nutmeg in a bowl.
Whip gently for 1 minute until the mascarpone is extended.
Add the Grand Marnier and whip at medium speed until soft peaks form.

Mix about ¼ of the mascarpone mixture into the chocolate to lighten.
Incorporate the remaining mascarpone until well incorporated.
Fill a piping bag with the mousse.

Mascarpone custard:
Heat the milk, 35% cream, and vanilla..
Work the egg yolks with the sugar until the mixture becomes foamy.
Pour a little at a time the boiling milk over the egg yolks while stirring vigorously.
Put the mixture in a saucepan and thicken the cream over low heat, stirring constantly.
The cream is cooked when it coats the spoon.

Add the mascarpone to the custard, mix well, and let cool.
Beat the 35% cream with an electric mixer until soft peaks form.
Fold the cream into the mascarpone cream.

Assembly:
Take the sides of a springform pan and place it on a serving plate.
Put a circle of meringue inside the pan and top with half of the chocolate mascarpone mousse.
Cover with raspberries, trying to arrange them aesthetically, in a single layer.
Cover with the second meringue, then the mascarpone mousse, then the raspberries, and finally the third meringue.
Press lightly with the palm of your hand.

Refrigerate for a few hours.
At serving time, decorate with raspberries, cut into pieces, and serve with the mascarpone custard.