Preheat your oven to 160℃ (140℃ if you're using a fan oven). Line an 8x8-inch tin with baking paper.
Break the white chocolate into pieces and put it in a heatproof bowl with the butter. Melt them together in the microwave, stirring every 20-30 seconds to prevent burning. Leave to cool slightly.
Add the melted mixture to a large bowl along with the caster sugar, light brown sugar, eggs and vanilla extract. Mix well.
Stir in all of the flour to make a smooth blondie batter.
Fold the white Twix pieces and the white chocolate chips into the blondie batter then transfer it to your lined tin.
Melt the caramel in the microwave for 10-20 seconds until runny then drizzle it over the top of the blondie mixture. Use a toothpick to swirl it around to make a pattern.
Bake for around 35-40 minutes, until golden brown all over with a slight wobble in the middle.
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!