I can't remember where I got this recipe, but it's good. [I've just changed the timings and heat - makes them gooier in the middle.]
200g dark chocolate
250g caster sugar
110g plain flour
half tsp salt
half tsp vanilla essence
Melt butter and chocolate in a glass bowl over boiling water.
Whizz eggs and sugar in the food processor till light and fluffy.
Add melted butter and chocolate and mix well. Sieve in flour, add salt and vanilla essence. Mix well.
Pour into a tray lined with baking parchment, bake for 20-25 minutes at 150C.
[You can use a sachet of vanilla sugar instead of essence. I sometimes add 125g chopped walnuts. Take care not to overcook.]