Preheat a fan forced oven to 170C / 340F (190C / 375F for no fan). 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 scoop (one that holds about 3 tbsp) 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.
StorageCookies can be stored in an airtight container for up to three days.Getting perfectly round cookiesUse a large cookie cutter to wiggle around each cookie to give it a rounder shape.UnderbakingUnderbaking your cookies results in a chewier cookie. That’s why you only bake them for 10 minutes in this recipe!Why don’t you chill the dough before baking?People typically chill their cookie dough for good reason. It helps the cookie ingredients meld together, resulting in an almost caramel-like cookie. It also results in a thicker cookie. For this particular recipe, you don’t need to do that. In fact, chilling them would result in a super tall cookie.