Cream together the butter, sugar and brown sugar until light and fluffy.

Blend in peanut butter, followed by the egg.

In a separate bowl, mix the flour, baking powder, and salt. Blend into the peanut butter mixture. Mix until smooth.

Shape into a log and chill until firm 30 minutes – 1 hour.

Pre-heat oven 350f/180c

Line cookie sheet(s) with parchment paper.

Cut into ¼ inch/5mm thick slices, arrange onto a prepared cookie sheet, and bake 9-12 minutes (until golden).

To make the Filling, heat the milk, brown sugar and the butter in a saucepan over medium-high heat. Bring to a boil.

Place the chocolate in a heatproof bowl. Pour the boiling milk mixture over the chocolate and let stand for 5 minutes. Whisk until smooth. Cover with plastic wrap and chill for at least 1 hour, or until it has thickened enough to be piped onto a cookie.

Once the cookies have cooled, match them into pairs by size. Turn one cookie of each pair over so that the flat side is facing up.

Place the filling into a piping bag with Pipe dots of the filling all around the flat face of each pair (as pictured). Top with the other cookie of each pair and press it onto the filling, so that it reaches the edges of cookies. Refrigerate for until set (about an hour) and then bring to room temperature to serve.