Preheat your oven to 190c/375f. Line a baking tray measuring approx 13"x7" with baking parchment and put to one side.
First make your crumble. Place the flour, baking powder and cold butter cubes in a large bowl. Rub the butter into the flour using your fingertips until the mix resembles breadcrumbs. Don't worry if you have some large chunks in there - it adds to the texture. Add in the white and brown sugar, the salt, egg and vanilla extract, then give it a mix with your hands. Take approx. half of this mixture and place it into your prepared baking tray. Flatten the mixture down and squash it to the edges and corners.
Take a medium sized bowl and add your strawberries, blueberries, sugar, lemon juice and cornstarch. Mix together gently and then layer this mixture on top of your crumble base. Sprinkle the remaining crumble mixture on top of the blueberries and place in the oven to cook for 40 minutes - until the topping looks golden brown.
Take out of the oven and leave to cool in the tin before gently sliding (baking parchment too) onto a cutting board. Leave to cool completely (this is a good time to make your custard dip). Once cool, slide out of the baking parchment and cut into 24 bars
Whilst the streusel bake is cooling, make the white chocolate custard dip.
Place the egg yolks, sugar and cornflour in a large bowl or jug and whisk. Put to one side.
Place the cream, milk and vanilla extract in a pan, and heat until it’s just about to boil. Turn off the heat and pour a splash of the hot milk mixture into the egg mixture whilst whisking with a hand whisk. Continue to add the milk a little at a time whilst whisking, then pour the whole mixture back in the pan. Stir slowy with the whisk whilst heating on a medium heat. The custard will start to thicken after a couple of minutes.
Once the custard has thickened, turn off the heat and stir in the butter. Place the chopped chocolate in the pan and leave for a minute or two to melt, then stir to fully incorporate.
The custard can be served warm right away, or can be left to cool with a layer of plastic wrap on top (the plastic wrap needs to be touching the whole surface of the custard to stop it forming a skin. Once cool, it can be refrigerated until needed.
Serve the sliced streusel bars with little individual dishes of the white chocolate custard dip.
The streusel bars will last in an airtight container for up to 5 days (they'll become less crunchy on top after a day or two - but still delicious)..Nutritional Information is per serving.