Mix the room-temperature butter with the sugar, flour and cocoa, and quickly knead into a dough.

Roll it out very thin, 2 mm thick, between two sheets of parchment.

Use a round cutter, 4 cm in diameter, to cut out disks right on the parchment and freeze for at least an hour.

Put all the ingredients except the flour and eggs into a saucepan and bring to a boil.

Add all the flour at once and cook the dough with a spatula: within a minute it should pull away from the bottom and sides.

Remove from the heat and let it rest for 15-20 minutes to cool.

Beat in the eggs one at a time, mixing the dough; add the last egg gradually until you reach the right consistency.

On parchment or a silicone mat, pipe round mounds 3-4 cm in diameter and top each with a disk of craquelin.

Bake in an oven preheated to 180 °C for 30-35 minutes, without opening the door.

Leave the finished pastries in the switched-off oven for 5-10 minutes, then remove and let cool.

Whip the mascarpone with the boiled condensed milk into a fluffy cream.

Cut the cap off each pastry, fill with cream and put the crisp top back on.

Serve right away.