Preheat the oven to 160°C/320°F (standard) or 150°C/300°F (fan forced / convection).
Grease and line an 8 inch (20cm) square pan with baking paper
In a large bowl, whisk the butter and sugar together until creamy. (about 3 minutes)
Whisk in the peanut butter until combined.
Use a spatula or wooden spoon to fold in the flour to form a soft dough.
Press the dough out into the prepared baking pan.
Pre-cut the dough into the serving slices you prefer and pierce each cut portion with a fork.
Bake for 20 - 25 minutes until lightly browned. See note 3
Once the shortbread is done baking, leave it in the pan and slice it along the pre-cut lines while it's still warm. Give it time to cool down in the pan before attempting to remove it; it will become firmer as it cools. See note 4
Dust with a sprinkling of castor sugar for a decorative look and some sweet crunch (optional)