Preheat the oven to 350 degrees Fahrenheit. Line a 9x13-inch baking pan with foil, being sure to leave some longer pieces over the edge for lifting later.
Spray the foil inside the baking pan with non-stick cooking spray. Set aside.
Place butter and sugar in the bowl of your stand mixer and beat until smooth and creamy.
Add the egg yolk and vanilla. Beat to combine.
With the mixer on low, slowly add the flour. Mix to blend well.
Scrape down the sides of the bowl. Make sure all the flour has mixed in to the dough.
Gently press dough into the bottom of the prepared pan, spreading evenly with your fingertips.
Place in the oven and bake until set and edges are golden, about 25 to 30 minutes. Depending on your oven, start checking after 20 minutes.
Remove the pan from the oven, cover the crust layer with the peanut butter and chocolate chips. Return to the oven for 4 minutes.
After 4 minutes, remove pan from the oven and spread the melted chocolate/peanut butter mix with an offset spatula. Smooth all the way to the edges of the pan.
Sprinkle the top with nuts. Cool in the pan on a wire rack for at least an hour.
When cool, remove from the pan by lifting up on the foil. Cut into bars or desired number of pieces.
Store in an airtight container for up to 3 days.
*NOTE: These can be frozen for up to 3 months too!