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.
Stir in the eggs and vanilla extract.
Add all of the flour and mix well to make a smooth, thick blondie batter.
Chop the Twix bars into chunks and fold them into the batter along with the milk and white chocolate chips. Transfer to your lined tin and spread out evenly.
Bake for 35-40 minutes, until the edges are pulling away and look golden brown. The middle should be lightly golden but still have a slight wobble.
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, drizzle some caramel over the top, cut into squares and enjoy!