First measure the dry ingredients into a bowl, then stir in the butter quite thoroughly.
Next sprinkle half the mixture over the base of the tin and, using your hands, press and even it out to form a base without any gaps. Then arrange the filling carefully all over this.
Sprinkle the rest of the mixture evenly over the filling and press this down firmly. Bake near the centre of the oven (or just above) for about 20–25 minutes until golden brown.
Leave in the tin for 10 minutes, then cut into 15 squares, cool on a wire rack and store in an airtight tin.
Note: these can be made with the same quantity of any other fruit. I particularly like dates but these need to be simmered in 6 tablespoons of water for about 10 minutes in advance. Or you can use fresh fruits – plums are good