Preheat oven to 160°C/Fan 140°C/Gas 3. Line 2 large baking trays with non-stick baking paper. Place the butter and both the sugars in the bowl of an electric mixer and beat on medium speed for 6–8 minutes or until sandy in texture.
Add the egg, extra yolk and vanilla, increase the speed to high and beat for 2 minutes or until pale and creamy. Place the baking powder, bicarbonate of soda and water in a small bowl and mix to combine. Add the baking powder mixture, flour, table salt, cocoa, milk and chocolate to the butter mixture and beat on low speed until just combined.
Working in batches, shape 60ml portions of the dough into balls and place on the trays, allowing room to spread. Bake for 18–20 minutes
or until golden brown and firm to the touch. Allow to cool on the trays for 10 minutes, before transferring to wire racks to cool completely. Repeat with the remaining dough to make a total of 15 cookies. Spread the cookies with the caramel and sprinkle with sea salt to serve. You can store them, without the caramel and sea salt, in an airtight container for up to four days.