Preheat the oven to 180°C and grease 4 small ramequins with butter.
Using an electric whisk, whisk butter and caster sugar in a large bowl until combined. Add the eggs one at a time and continue to whisk until pale and frothy. Add the vanilla extract and whisk again.
Fold in the gluten free flour and melted dark chocolate.
Divide the mixture between 4 ramequins and bake in the oven for 10 minutes for a runny middle and 12 minutes for a slightly more cakey middle. Serve warm.