Preheat oven to 180 degrees, melt chocolate and leave to cool.
Beat butter and sugar slowly until combined, then start to beat the mixture on fast until white/pale/fluffy.
Add your beaten egg a little at a time on slow until each bit is incorporated
Pour all the cooled choc into your mix, beating all the time. Stir in the vanilla extract. Fold in the plain flour.
Spoon evenly into the tin and bake for 30-40 minutess at 160C (fan). Test your bake with a knife; this should come out with a slight mark of the cake mixture as we are trying to achieve a gooey consistency.
Once cooled mix until a fine crumb consistency is achieved. Mold into balls ready to decorate