Preheat the oven to 180℃ and line a loaf tin with parchment
Cream together the margarine and caster and soft brown sugars until light and fluffy
whisk in the 1 tsp milk and vanilla extract
Add the flour, bicarbonate of soda and salt and combine to form a lovely soft dough.
Divide into two portions (each will be about 190g)
To one half stir in the additional 1 tsp milk and the cocoa powder
First, spread the chocolate cookie mixture onto the base of the lined tin
Next, place the vanilla cookie layer on top and smooth over to make a nice even layer. (This is easiest by breaking the dough into small pieces and dotting over the chocolate layer and then smoothing over)
Prick all over with a fork and place in the oven for 20-25 minutes until risen and golden. Leave to cool.
Meanwhile, make the strawberry icing layer.
Blitz the freeze dried strawberries (keeping 2 aside) with 100g of the icing sugar to make a strawberry sugar. (leave out this step if using strawberry powder and just stir into the 200g icing sugar instead)
Whisk the margarine, icing sugar and strawberry sugar until you have a thick, but smooth buttercream
Spread over the cooled cookie base and level off.
Decorate with finely grated freeze-dried strawberry.
Place in the fridge for at least half an hour to firm up
Cut into 8 even squares. These cookies are best kept in the fridge and eaten within 3 days