Preheat the oven to 180°C / 160°C fan / gas mark 4 and line a square baking tin with baking paper.
In a large bowl, mix together the plain flour, cocoa powder, light brown sugar, baking powder, and salt.
In a separate bowl or jug, whisk together the vegetable oil, dairy-free milk, vanilla extract, and melted dairy-free dark chocolate.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Fold in the dairy-free chocolate chips if using.
Spoon the mixture into the prepared tin and spread it out evenly.
Bake for 22–25 minutes until the top is set and the centre still has a slight softness.
Leave to cool in the tin before slicing into squares.