Preheat the oven to 200°C (392°F).
Cut the butter into cubes. In a large food processor, combine the butter, flour, sugar, baking powder, and salt. Mix until well combined. (Or use your hands to quickly rub the butter into the dry ingredients until the mixture forms coarse crumbs.)
Line a 26 x 36 cm (10x 14 inch) baking pan with parchment paper for easy removal.
Press half of the crumble mixture into the prepared baking pan. Bake for 10 minutes, then remove from the oven.
While the crust is baking, mix the milk into the pudding powder in a mixing bowl using a hand mixer. Add a little extra vanilla extract, stir, and let it set for a few minutes.
Spread the vanilla custard filling on top of the partially baked crust.
Evenly spread the blueberries and raspberries over the vanilla filling. Fresh berries are preferable to frozen ones as they tend to be less watery.
Then sprinkle the remaining crumble mixture on top to cover well.
Bake for 25-30 minutes or until the top crumbs turn golden brown.
Allow the bars to cool completely in the baking pan.
Finally, refrigerate the baking pan with the bars for at least 1 hour. This step is crucial to prevent the bars from breaking.
Cut into squares and dust with icing sugar before serving.