Place the sugar and egg in a large bowl and whisk together briefly until combined. Add the vanilla and salt and again whisk briefly to combine. Now add the peanut butter and whisk until a thick but smooth dough is formed. Cover the bowl and refrigerate for 30 minutes.
Line a baking tray with parchment paper. Using a 60ml mechanical ice-cream scoop or your hands, place 6 scoops or balls of cookie dough on the prepared tray, spacing them well apart. Transfer to the freezer for 10 minutes.
Meanwhile, preheat the oven to 180°C/Fan 160°C/Gas 5. Sprinkle the chilled cookies with sea salt flakes and bake for 20–22 minutes, or until the cookies have spread a little and the edges are lightly browned. Set aside to cool completely.
To serve, dip the cookies halfway into the melted chocolate and then place back on the lined tray. Refrigerate until the chocolate has set. If stored in a sealed container, these cookies will keep for 4–5 days.