Thanks for your answers!
In France, places with air conditionning are very rare and I don't have that in my work kitchen nor at home. My work kitchen is rather fresh at the moment, we will see if it lasts(it is 91° F today outside).
The problem is when I deliver cookies or ship them, I have a wedding next week and I'm affraid the cookies are going to wait in a place with no air conditionning.
I could try putting a royal icing layer but RI is porous so I don't think it will stop the butter to go through.