1. Mix together in a robot the egg, sugar, salt, vanilla, then add the butter in pieces and mix again.

2. Add the coffee and mix again.

3. Transfer the butter cream into a piping bag with a fluted nozzle.

4. Create regular waves on one of the boudoirs.

5. Stack another boudoir, pressing slightly to hold it in place.

6. Continue with the rest of the boudoir biscuits.

7. Refrigerate for 45 minutes before serving.