Preheat the oven to 180°C (350°F) and line an 8"x8" baking tin with baking paper.
Melt the butter and white chocolate together.
Beat the brown sugar, sugar, egg and vanilla extract together until light and fluffy.
Mix in the chocolate mixture.
Stir in the flour and baking soda (and chocolate chips if you are adding those).
Add in the mint extract and few drops of green food colouring and pour in the baking tin.
Bake for 25-30 minutes.
Should be lightly golden but still soft in the middle with light crumbs attached when a skewer is inserted.
Notes
Be very careful with the green food colouring. Start with one or two drops and mix and add more if you want more green. Add extra peppermint extract if you like a strong mint flavour.