Cream the butter and the sugar in a bowl of an electric mixer fitted with a paddle attachment.
Add the whole egg, beat for about 2 min. Scrape down the sides of the bowl and beat again.
Add the yolk, beat for about about 2 min. Scrape down the sides of the bowl and beat again.
Add the vanilla and the peanut butter, mix until combined.
Add the flour, beat until just combined. Do not overmix.
Half the dough and shape into two large sausages, wrap in clingfilm, and chill in the fridge for an hour.
Preheat the oven to 160°C and line two baking trays with baking paper.
Remove the dough from the fridge and slice using a sharp knife into ½cm thick circles. Place the cookie rounds onto the baking paper and sprinkle with the chopped peanuts, press them into the dough slightly.
Bake in a preheated oven for about 15 min or until lightly golden, enjoy!