Place 8 tsps of peanut butter onto a lined baking tray in little separate dollops. Freeze for 1 hour.
Start making the cookie dough while this is freezing: In a large bowl, using an electric/stand mixer (or hand whisk, just double/triple the times) combine the butter and both sugars on medium speed for several minutes, until fully combined. Add the egg and vanilla and mix again.
Add the flour and baking powder and fold in using a rubber spatula or wooden spoon, until just combined. The dough should be fairly solid at this stage (it ought to almost form a ball, albeit a very soft one!). Add the chocolate and stir together.
Let this dough chill in the fridge for at least 30 minutes.
After the PB has had at least 1 hour in the freezer, and the cookie dough at least 30 mins in the fridge: Preheat oven to 200C/180C Fan/Gas Mark 6. Line 2-3 large baking trays.
Scoop large balls of cookie dough – somewhere between golf and tennis balls, so you have 9 of similar size. Place on the trays, with several inches in between (max 3 per tray).
Make an indentation with your thumb into each ball of dough. Place a little ball of peanut butter. Wrap the dough tightly around the peanut butter so none will leak out.
Bake for 10-12 minutes, until just golden at the edges. They may look underbaked in the centre but they keep cooking from the residual heat, and this gives them their perfect soft texture. Repeat for all cookies and leave to cool a little on a wire rack before serving (although eating them whilst still a little warm really is the best).