Preheat the oven to 180°C (350°F) and line a 9"x9" baking pan with baking paper.
Sprinkle over the white chocolate buttons in an even layer over the base of the baking pan and set aside.
Melt the butter, milk chocolate and dark chocolate together.
Place the eggs, vanilla extract, brown sugar and sugar in a bowl and beat together.
Pour in the chocolate mixture into the egg mixture and mix together.
Add in the cornflour and cocoa and mix until all combined.
Pour over top of the white chocolate buttons and place in the oven for around 30 minutes. The top and sides will be set and the middle will be slightly wobbly.
Notes
You could use all dark chocolate for a richer brownie.