Preheat your oven to 180°C and grease and line a 20cm square baking tray.
Add your dairy free butter, light brown sugar and granulated sugar to a large mixing bowl and whisk with an electric whisk until light and fluffy.
Add your egg and continue to whisk. Add your vanilla extract and whisk again.
Add your self raising flour, cornflour and sea salt and mix well.
Break your chocolate (100g dairy free milk chocolate and 50g dairy free white chocolate) into small pieces and fold into the mixture.
Transfer the mixture to your prepared baking tray, ensuring it's pressed into all the corners. Bake for 20-25 minutes, until golden brown on the top. Mine took around 22 minutes.
Remove from the oven and leave to cool in the tin, once cool melt your extra dairy free chocolate and drizzle on the top. Allow the chocolate to solidify and cut into squares.