Preheat the oven to 180C/350F and spray a 10 or 12 inch cast iron skillet with cooking spray.
Make the fruit filling; place the berries, flour and sugar into a medium sized bowl and mix to coat all the berries in the sugar/flour; leave to one side.
Make the cobbler topping: Place the flour, cocoa powder, sugar, baking powder and salt into a large bowl and mix briefly.
Pour in the melted butter and add in the eggs. Stir with a wooden spoon/spatula until the dough comes together in one big ball. It should be a little sticky and come away from the sides of the bowl.
Tip the berry mixture into your prepared skillet and smooth into an even layer.
Take a 1.5tbsp cookie scoop and place rounds of the cobbler topping on top of the fruit, it's fine if you don't cover it completely and can still see some fruit peeking through.
Place in the oven for 25-30 minutes, until the fruit is bubbling in the centre and the cobbler topping feels springy to the touch and looks matte in the light of the oven.
Leave to cool for at least 15 minutes if you're going to serve it hot, the inside will be molten. It's best served warm, so I let mine cool for 30 minutes.
You can either place the skillet on the table and just grab a spoon, or divide it into bowls. Serve with ice cream or custard.
Chocolate Mixed Berry Cobbler will keep in the fridge for 3 days. Reheat it in the microwave for 1 minute to serve. I would advise, if you're not going to eat all of it on the day, that you take it out of the skillet and transfer it to an airtight container to store in the fridge otherwise it may start to taste like metal.