Preheat oven to 350F (175C)

Prepare a 9x9 pan by lining with aluminum foil or parchment paper -- use enough so that it hangs over the edge of the pan (this will make it easy to lift the finished bars out of the pan).  If using aluminum foil, lightly butter the foil.  Set aside.

In KitchenAid or with electric hand mixer, cream together butter and peanut butter.

Add sugars and beat sugars until well-combined.

Add egg and vanilla extract, stir well.

In a separate bowl, whisk together flour, baking powder, and salt.

Gradually, on low speed, add flour mixture to peanut butter mixture, pausing occasionally to scrape down sides of bowl.

Press half of the peanut butter bar batter evenly into the bottom of your prepared pan.  Set aside the remaining batter while you prepare your fudge.

Chocolate Fudge

Combine chocolate chips, condensed milk, and butter in a medium-sized saucepan over medium-low heat.

Stir frequently until chocolate chips are completely melted and mixture is smooth.  Remove from heat and stir in vanilla extract and salt.  Allow to cool for 5 minutes.

Pour chocolate mixture evenly over your prepared peanut butter blondie base.  Sprinkle fudge with remaining fudge mixture.

Bake on 350F (175C) for 23-25 minutes.  If desired, sprinkle with flaky sea salt while still warm.

Allow to cool completely before cutting and serving.  Because the fudge will need to set, it may need to set as long as 4-6 hours before the fudge is completely firm.