Line a 9x13" baking pan with aluminum foil so that the foil extends 2" beyond the short ends of the pan. Generously spray the foil with non-stick cooking spray; set aside.
In a medium-sized bowl whisk together the flour, baking powder, and salt; set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a hand held mixer, beat the butter and peanut butter on medium-speed until smooth, creamy, and completely combined; about 2 minutes. Add the brown sugar and continue beating until light and fluffy; about 2 minutes. Add in the eggs, one at a time, beating well after each addition. Beat in the vanilla. Reduce the mixer speed to low; add in dry ingredients and beat until just combined. Turn mixer off and stir in reese's pieces and chocolate chips with a rubber spatula.
Using the rubber spatula, scrape the batter into the prepared baking pan and smooth the top. Press pretzels lightly into the top of the batter, and add a few extra chocolate chips/reese's pieces if desired.
Bake the bars for 35 minutes, or until they are golden brown and almost completely set. They will firm up quite a bit as they cool, so be sure not to over bake.
Cool completely in the pan before cutting in squares and serving.
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