I have a need to eat toffee cupcakes with runny toffee centres. I've got a can of condensed milk on to boil and am planning on doing a normal toffee cake mix and putting a spoonful of cake mix, then a spoonful of toffe, top with a spoonful of cake mix, bake as normal. Does this sound like it would work?
Not sure on what icing would be best. Hmm . . . maybe just more toffee?? 
Any suggestions?