For the peanut butter cookie base: Preheat the oven to 350 degrees F with a rack set on the middle shelf. Lightly butter a 9- by 9-inch baking dish.
Whisk together the flour, baking soda and salt in a medium bowl.
Combine the butter, peanut butter and brown sugar in another medium bowl. Beat with a hand mixer until creamy, about 2 minutes. Add the egg and mix well. Add the dry ingredients and beat until just combined. With lightly floured hands, press the dough into the prepared baking dish and bake until lightly browned, 15 to 18 minutes. Place the baking dish on a cooling rack and cool completely.
For the chocolate sour cream frosting: Place the chocolate chips in a medium heat-proof bowl. Microwave in 30 second intervals until the chocolate is melted. Set aside to cool slightly.
Whip the butter until light and fluffy with a hand mixer. On low speed, add the confectioners' sugar, 1/2 cup at a time, and whip until all of the sugar has been added and the mixture is smooth and creamy. Add the melted chocolate and sour cream and mix until well combined.
To assemble: Spread the chocolate sour cream frosting evenly on top of the peanut butter cookie base. Top with the crushed peanut butter candies, potato chips and pretzels. Cut into 16 bars and use a small offset spatula to remove the bars from the pan.