Measure leftover cakes and add some crushed cookies to it in case there's not enough leftovers. Put it in a medium size bowl to mix.
Dice up and soften butter if necessary. Take butter out of the fridge half an hour before use or put it in a microwave for a couple of seconds to be soft enough to work with easily.
Depending on what the leftover cake tastes like put unsweetened cocoa powder into the bowl as well. In our case it is a chocolate based (Sacher torte) leftover which is originally made with cocoa powder and chocolate so it only needed 1 tbs cocoa powder. In any other non-chocolatey leftovers don't hold back and put at least 2 tbs cocoa powder into the bowl followed by the jam of preference, butter, raisins and rum aroma.
Mix it until all ingredients are evenly placed in the dollop.
Flip the base upside down onto a wire rack and place the wire rack onto a parchment paper.
Mark the pieces out. Cut the base at the markings.
Melt chocolate for icing.
Pour melted chocolate on top of the pastry.
Spread melted chocolate evenly.
Put rack in the fridge for half an hour. Let the chocolate settle.