Mix the puree, extract and cardamom together. Stir in icing sugar (this will become difficult and you will eventually need to knead it in like bread dough, it'll be quite stiff). Portion out the filling into the desired size for chocolates and place them in an airtight container in the freezer while you prepare the coating.
Mix the chocolate and shortening in a glass bowl and microwave for short bursts of 20-30 seconds, stirring at each interval until the chocolate is just melted. It should not be a hot mixture at all. Using skewers or forks, dip the watermelon centres in the chocolate, drain off excess chocolate and place on parchment paper to set fully.
Yield: 2-3 dozen small chocolates