The filling
Crumble cooled cake finely and add frosting a spoonful at a time until the mix just holds a shape when pressed. Too much frosting makes it heavy and greasy, and it will push the shell apart as it settles.
Coating the mould
Spoon melted coating into the cavity and push it up the walls with the back of a spoon or a small brush, keeping the layer even. Thin spots are where the shell breaks on release; a second coat after the first sets is the usual fix.
Stick and closing
Set the stick into the coated cavity while the shell is still tacky, press in the filling without overpacking, then cap with more melted coating and scrape the surface flat. A flat base is what lets it stand and what makes the seam invisible.
Chilling and releasing
A short chill sets the shell; a long one makes it brittle and prone to condensation when it comes out. Release by flexing the mould rather than pulling the stick.