Preheat the oven to 350 degrees F and line 1-2 baking trays with parchment paper.
Make the cookies: stir together the peanut butter, sugar and vanilla until smooth.
Add in the oat flour, baking powder and salt and stir to a thick, sticky batter.
Divide into 12 balls and place each cookie ball on the tray(s) with a little room around each one. Use a fork to press a criss-cross pattern onto the top of each cookie.
Bake for 12-14 minutes until golden and crisping at the sides. Allow to cool for 20 minutes before moving the cookies as they will be delicate then cool fully on a wire rack.
Once cool, make the chocolate filling: allow the cream cheese to rest at room temperature so it's not too cold then beat until smooth with the icing sugar.
Melt the chocolate then allow to cool down (so it’s not too hot) and once cool, quickly stir into the cream cheese mix. The frosting will start to thicken as it cools, so work quickly. If it becomes too hard, warm up for a few seconds in the microwave.
Transfer the mix to a piping bag with a star-shaped nozzle and pipe on the underside of 6 cookies, leaving a small gap in the middle. Fill the gap with the peanut butter and sandwich together with the other cookies.
Eat straight away or keep in a sealed container at room temperature for 3-5 days. the cookies alone will keep for 5-7 days and can be frozen for up to 1 month.