Preheat the oven to 180C/gas 4/ fan 160C. Grease and line the base of a 20cm round cake tin.
Put the lactose free butter, sugar, eggs, gluten free flour, baking powder, cocoa, cinnamon and lactose free milk into a bowl.
Beat with a wooden spoon for 2-3 minutes, or with an electric hand mixer for 1-2 minutes, until light and fluffy.
Tip the mixture into the tin then spread it level. Bake for 1 hour to 1 hour 10 minutes, until risen, nicely browned, feels firm to touch and springs back when lightly pressed (cover with foil if it starts to brown too quickly).
Cool in the tin for 10 minutes, then turn out, peel off the paper and cool on a wire rack. When cool sprinkle with icing sugar, then serve.
Wrap tightly in double thick foil to keep fresh for up to a week.