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)
Storage
Store at room temperature in an airtight container for up to 7 days.
Recipe Notes
1.If you are using peanut butter with added sugar, I suggest you cut down on the sugar. Or else the shortbread will be too sweet. 2 - 3 Tablespoons ( 25g - 37g) should be sufficient when using peanut butter with added sugar.2. Peanut butter: Crunchy or smooth peanut butter works well in this shortbread. I used a sugar free peanut butter for this recipe. A ¼ cup of sugar added the perfect sweetness to the shortbread.3. Crisp or soft - The baking time suggested in the recipe above is for a crisp shortbread. Reduce the baking time by 5 minutes If you prefer a softer shortbread.5.5. Cool before removing from the pan - Allow the baked peanut butter shortbread to cool in the pan before taking it out. It will become more firm as it cools. When warm, the shortbread is still delicate and can easily break if removed too early.