To prepare the base crust, grind the crackers into crumbs.
Add sugar and butter, stir.
Put the mass into a mold with a diameter of 22 cm, tamp well, forming also small sides.
Bake the biscuit cake in the oven for 10 minutes at 150 degrees. Refrigerate.
Put chocolate and marshmallows in a heavy-bottomed saucepan, pour in the milk. Cook over low heat, stirring constantly until the chocolate is dissolved. The mass should be completely homogeneous and smooth. Refrigerate.
Add the chocolate mass to the whipped cream and stir.
Place the chocolate mousse on the cake base and spread evenly.
Chill the dessert with chocolate mousse in the refrigerator (about 3 hours). You can decorate the finished dessert with chocolate pieces.