Preheat the oven to 180°C / 160°C fan / gas mark 4 and line a medium traybake tin.
In a large bowl, mix together the flour, cocoa powder, sugar, baking powder, and bicarbonate of soda.
Add the dairy-free milk, vegetable oil, and vanilla extract, then stir until smooth and fully combined.
Fold in the dairy-free chocolate chips if using.
Pour the mixture into the prepared tin and spread it out evenly.
Bake for 22–25 minutes, or until the traybake is risen and a skewer inserted into the centre comes out clean.
Leave to cool in the tin before slicing into squares.