Peel the prawns and remove their heads, putting the flesh on a plate and the shells and heads in a small saucepan. Pour the cream into the pan and bring to the boil, crushing the shells in the cream with the back of a spoon as you go.
Turn off the heat once the cream comes to the boil, then cover and leave to infuse for 30 minutes.
Cut the prawns into short pieces. Strain the cream through a sieve, discarding the shells. Pour the cream back into the saucepan, add most of parmesan and season with black pepper.
Leave to simmer for 5 minutes until the cream has thickened slightly, then stir in the dill and the chopped prawns.
Get an oven grill hot, toast the bread on either side, then spoon the prawn mixture over the toast, dust with the reserved parmesan and slide under the hot grill till just bubbling and starting to colour. Serve immediately.