1st batch : Combine 100gm butter with 120gm dark brown Muscova sugar, 120gm plain flour and 1teaspoon ground cinnamon. Mix together till mixture resembles wet sand and pad into baking tin to bake blindly.
Make 2nd batch of the above. Throw in blueberries and raspberries into baked tart shell and put 2 tablespoon of castor sugar over fruits. Sprinkle 2nd batch of mixture all over fruits till fully covered. Bake again till toppings are crispy and lightly brown. Cool and refrigerate for 3 hrs for easy cutting.