Preheat your oven to 160℃ (140℃ if you're using a fan oven). Line an 8x8-inch tin with baking paper.
Break the Milky Bar 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 for a few minutes.
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 Milky Bar buttons and the white chocolate chips into the blondie batter then transfer it to your lined tin.
Bake for around 35-40 minutes, until golden brown at the edges 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!