Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch baking pan with parchment paper or foil. Set aside.
Whisk together the butter and sugars until smooth and combined. Add the egg, egg yolk, peanut butter, vanilla, and salt, and whisk until smooth and combined. Fold in the flour until completely combined. The mixture will be quite thick.
Spread the batter evenly into the prepared pan, and bake for about 25 - 30 minutes, or until the centre is no longer jiggly and a toothpick inserted comes out clean. Do not over-bake. Allow to cool completely in the pan on a wire rack before topping - about 2 hours.
For the Topping
In a microwave-safe bowl, add the chocolate chips and peanut butter. Heat in 30 second intervals, stirring after each one, until melted and smooth. Fold in the chopped peanuts.
Spread the melted chocolate over the top of the blondies, sprinkle with extra chopped peanuts if desired, and allow to set in the fridge before slicing into bars - about 1 hour.
Notes
Blondies can be stored in an airtight container at room temperature or in the fridge for up to 1 week. They also freeze well for up to 3 months. Thaw overnight before serving.