In a small saucepan, bring the butter, water, salt and sugar to a boil.
Remove from the heat, let cool for a few minutes and stir in the flour and baking powder.
Form a ball, cover with a damp cloth and place in the fridge for two hours.
Preheat the oven to 180 ° C.
Roll out the dough between two sheets of parchment paper to 3 millimeters thick then cut similar shapes in the dough and place on a plate covered with parchment paper until there is no more dough.
Bake for about 12 minutes.
Let the cookies cool on a wire rack (or on the baking sheet).
Apply a generous amount of milk jam to a cookie and cover with another cookie.
Repeat the operation until all the ingredients are used up. The jam will harden and dry a little which will make it easier to taste the cookies. Keep for a few days in an iron box.