Preheat oven to 350 degrees. Spray a 9 x 9 inch baking pan with nonstick cooking spray and line with parchment; set aside.
In a large bowl, use a handheld mixer to beat together the brown sugar, butter, ¼ cup peanut butter, egg, and vanilla until well combined. Add in the flour and mix until just combined.
Pour the mixture into the prepared baking pan and press it into the pan. Bake for about 20-25 minutes or until the shortbread just starts to brown. Place on cooling rack and let cool completely before adding icing.
To make the icing, combine the chocolate chips and ½ cup of peanut butter in a microwave safe bowl. Microwave at half power (50%) stopping to stir every 30 seconds, just until the mixture is melted. Spread the icing on the cooled bars and top with crushed peanuts if using. Let the icing harden completely before removing the bars from the pan and cutting into squares.
This is a shortbread bar, so the base will not be super sweet and will be slightly crumbly.
Make sure you are using room temperature butter and peanut butter so that everything gets incorporated smoothly.
You can use either smooth or crunchy peanut butter depending on your preference.
Make sure the icing is completely cooled before cutting these into squares. You may want to refrigerate for a couple of hours before cutting.
Store these in an airtight container in the refrigerator for up to a week. Can also be frozen for up to a month.