1. Grease an 8-inch-square baking sheet, then line the bottom with parchment paper and grease the paper.
2. In a pot, combine butter, peanut butter and vanilla and place over medium-low heat. Stir constantly while heating, until everything is combined, then remove the pot from heat.
3. Add sifted icing sugar to the peanut mixture, working in batches by making sure all of the sugar is incorporated before adding more sugar. When all of the sugar is incorporated, spoon the mixture into the greased pan and place in the fridge for at least two hours before cutting.