Try melting the chocolate in a bain marie (bowl set over a pan of boiling water on hob) Melting chocolate in a pan directly on hob is usually a disaster.
Alternatively chuck straight into a pan all together: 4oz butter, 4 tbsp golden syrup, 4tbsp icing sugar and 4dsp cocoa powder and melt together and add corn flakes/rice krispies etc. It won't curdle/split/go hard etc and is the best base for chocolate crispy cakes/biscuit cakes. Everyone goes crazy for it. For a load of cakes for a party probably double the recipe.