Preheat oven to 350 degrees F. Line a 9 inch square metal pan with foil and spray foil with cooking spray.
Thoroughly stir together flour, baking powder and salt. Set aside. If you don't have a scale be sure to measure your flour with a light hand or by spooning it into the cup.
In a mixing bowl, beat together butter, peanut butter and both sugars. Add eggs and vanilla and beat on low speed just until blended (do not whip). Add flour mixture, stir until incorporated, then spread in pan. To spread evenly, dampen your fingers with some water and tamp down on the batter to flatten.
Prepare chocolate swirl. Melt chopped chocolate and butter together in the microwave on high stirring every 30 seconds. Drop by spoonfuls on top of peanut butter batter and drag a knife through the batter to marble. The chocolate mixture will be sitting on top of the batter and sinking in a little.
Bake on center rack for 25 minutes or until edges appear brown and center appears set. Do not overbake.
Allow the bars to cool slightly while you make the peanut butter topping. To make the peanut butter topping, stir together peanut butter and melted butter. Spread this mixture over the warm (not piping hot, not cool) brownies. Let cool for about half an hour.
Prepare Chocolate Swirl. Put chocolate in a heavy duty freezer bag. Microwave the bag for 30 seconds. Knead it with your hands to melt chocolate. Repeat, microwaving another 30 seconds and kneading bag, until chocolate is completely melted. Make sure there are no lumps. Snip a tiny hole in bottom corner of bag and drizzle chocolate decoratively over peanut butter. Or skip the bag, melt the chocolate in a custard cup and splatter across top with a spoon.
Decorate with peanut butter cups and candies if you want -- especially if you used crunchy peanut butter.
Put the brownies in the freezer for about 10 minutes or just until chocolate drizzle is set.
When set, lift from pan and slice brownies in to squares or bars. If they're too firm to slice, let them sit at room temperature Let come to room temperature before eating. Store covered and at room temperature for softer brownies.