Preheat the oven to 180C/350F/gas mark 4. Grease a 15cm/6inch cast iron skillet or an oven-proof dish.
Mix together the melted vegan butter, light brown soft sugar and caster sugar then stir in the vanilla extract and soy milk.
Add the plain flour and the baking powder and stir until smooth. Mix in all but a small handful of the chopped chocolate.
Transfer the cookie dough to the prepared skillet and spread level. Scatter over the reserved chopped chocolate. Arrange the raspberries on top and press them gently into the dough.
Bake the skillet cookie for about 15 minutes, until it is golden and crisp around the edges but still soft in the middle. It will firm up as it cools so be careful not to over-bake; you want the middle to be a little soft and gooey.