Preheat your oven to 160℃ (140℃ if you're using a fan oven). Line an 8x8-inch tin with baking paper.
In a large bowl, mix together the melted butter, caster sugar and light brown sugar. Then stir in the eggs and vanilla extract.
Add the flour and the custard powder. Mix well to make a smooth blondie batter.
Fold in the Custard Cream biscuits and the white chocolate chips.
Transfer the batter to your lined tin and spread out evenly. Bake for around 35-40 minutes, until they're golden brown and pulling away from the tin slightly (they will still be a bit gooey in the centre when you first take them out).
Leave the blondies in the tin to cool completely, then put them in the fridge for 3-4 hours to set. This will make the gooey centre turn firm and fudgy.
Once set, cut into squares and enjoy!