Heat the oven to 160°C fan/gas 4 and line the tin with baking paper. In a jug, whisk together the soy milk, coffee, chia, vanilla and lemon juice. Set aside for 10 minutes.
Put the chocolate and oil in a heatproof bowl set over a pan of simmering water (ensure the base of the bowl doesn’t touch the water). Heat until melted, stirring occasionally, then set aside.
Sift the flour, cocoa powder, bicarbonate of soda and caster sugar together in a large bowl. Pour in the milk mixture and melted chocolate with a pinch of salt, then stir well until combined and pour into the lined tin.
Bake for 30-35 minutes until the brownies have set but still have a wobble in the middle. Allow to cool completely in the tin for at least an hour, or ideally overnight. Cut into squares and serve with a dusting of icing sugar.