Preheat oven to Gas 6 200c.
Spray a 9-inch square tin with cooking oil, line it with baking parchment, and leave some hanging over the sides to help you remove the bars.
To make the dough mix the flour, sugar, baking powder and salt in a mixing bowl.
Rub the butter into the dry ingredients, it will look like breadcrumbs.
Add the beaten egg and mix with a fork.
Massage the dough with your hands to bring it together, it will be crumbly.
Scoop out 170g of the dough and set it aside.
Press the remaining dough into the tin.
In a mixing bowl combine, the fruit, sugar, cornflour and lemon juice.
Spread it out evenly over the dough base.
Now sprinkle the remaining dough that was set aside over the fruit.
Bake in the oven for 40-45 minutes at 200c Gas 6.
Allow to cool before removing from the tin and cutting into slices.
Serve with custard, cream or ice cream.