Preheat a fan forced oven to 180C / 350F. Line a large baking tray with baking paper. Set aside.
Add both flours to a large bowl along with the baking powder. Use a whisk to combine.
To a separate bowl add the softened butter, brown sugar, caster sugar, salt and vanilla extract. Use an electric beater to beat until pale and smooth, about 3 minutes.
Add the egg and egg yolk and beat until well combined. Add half the dry ingredients and 2/3 of the chopped chocolate and use a spatula to mix. Then add the remaining flour and repeat.
Use a large cookie cutter to scoop out 8-10 cookies. Bake for 10 minutes.
Once baked, top with chopped chocolate. Use a large cookie cutter to wiggle around each cookie to give it a rounder shape. Finish with a sprinkle of sea salt flakes.