Preheat the oven to 180°C and line a baking tray with baking paper.Add the butter and sugar to a mixing bowl and use an electric whisk to cream together until pale and fluffy. Add the peanut butter and mix until combined.
Mix in the egg and vanilla extract until smooth.
Add the flour and baking powder, then mix everything together until a soft cookie dough forms
Roll the dough into 1-inch balls using your hands. Place them onto the prepared baking tray, leaving space between each cookie as they will spread slightly.
Use a fork or potato masher to gently flatten each cookie and create a pattern on top.
Sprinkle a little caster sugar over the cookies, then bake for 10 minutes
The cookies will be very soft when they come out of the oven, so allow them to cool and firm up on a cooling rack. While the cookies are cooling, melt the chocolate using a bain-marie. Bring a small pan of water to a gentle simmer, then place the chocolate in a heatproof glass or metal bowl over the pan, making sure the bowl does not touch the water. Stir until melted and smooth
Dip each cooled cookie halfway into the melted chocolate, then place back onto the cooling rack. While the chocolate is still warm, sprinkle with a little sea salt. Leave to set before serving.