Device for preparing a cosmetic composition, kit and associated method

A device includes a base container delimiting an inner space receiving a first cosmetic product. It includes a compression wall, mounted so as to be slideably mobile in the inner space through an insertion aperture between an outer position and an inner position. The compression wall delimits through-passages for circulation of the first cosmetic product, each through-passage opening out into the inner space. The device includes a receptacle for mixing the cosmetic composition including the compression wall and a side wall displaceable together with the compression wall. The side wall protrudes from the compression wall and delimits a mixing space into which the through-passage opens out.