Preheat oven to 350°F. Spray an 8x8 square baking pan with non-stick cooking spray, line with aluminum foil and spray again. Set aside.

Mix butter, sugar, egg yolk and vanilla. Stir in flour and salt until dough is well blended. Spread into prepared pan. Bake 18-22 minutes or until lightly browned on edges. Let cool.

In a mixing bowl, combine melted butter, peanut butter, vanilla and powdered sugar. Beat until combined. Spread over cooled bars.

In a medium bowl, whisk together the powdered sugar and cocoa powder. Set aside. Place butter in large bowl and beat with an electric mixer on medium speed for 2 minutes. With mixer on low speed, gradually add half of the sugar and cocoa powder mixture, then half the cream. Beat on low until combined. Add remaining sugar/cocoa mixture and cream. Add vanilla. Beat on low until combined. Turn speed up to medium high and beat for 2-3 minutes. Frosting should be creamy and slightly shiny. If it's dull looking and too thick, add a little more cream. Try 1-2 more tablespoons cream, then beat again on medium for about 30 seconds. Spread frosting over peanut butter filling.

Slice and serve bars. Store covered at room temperature for one day or in the refrigerator for up to 5 days.