Preheat the oven to 200°C/400ºF/Gas mark 6

Line a shallow 20–22cm round cake tin or flan dish with baking/parchment paper

Put the cubed butter and caster sugar into a medium mixing bowl and, using a hand-held electric whisk, combine until light and fluffy

Add the egg before sifting in the flour, cornflour and baking powder. Combine until just mixed

Press the dough evenly over the bottom of the tin/dish

Set aside the remainder of the dough, covering it with cling film

Trim & slice the rhubarb before putting it into a large, non-stick frying pan

Sprinkle over the Demerara sugar

Put the pan over a low heat shaking the pan occasionally, until fruit is almost tender

Add the blackberries & blueberries to the rhubarb, gently mix for a few seconds before turning off the heat and allowing the fruit to cool

Spread the lukewarm fruit over the shortbread base before carefully dabbing pinches of the leftover dough over the top, allowing a little of the fruit to peep through

Bake for 30 minutes or until the shortcake top turns golden brown

Remove from oven and allow to cool for 10 minutes before turning out onto a wire cooling rack and then onto a serving plate

Sprinkle the top with icing sugar, slice and serve!