Filling (here, butter cream) should be soft enough to spread without tearing crumbs from the cake. If it is too thick to spread, warm over a water bath or dilute slightly. The cake will be easier to handle while adding filling if placed on a round of cardboard.
Slice the cake into horizontal layers. To moisten the cake, brush the cut surface with sugar syrup or liqueur.
Spread the filling evenly almost to the edge of the cake with a metal spatula. The thickness depends on the richness of the filling.
Set the next layer on top with the help of a cardboard round to move it. Press gently with the flat of your hand to squeeze the filling out just to the edge of the cake.