1) Preheat the oven to 350°F. Line a 9" x 13" baking pan with parchment, leaving the ends long to create a sling. This will make removing the bars easier.
2) To make the crust: Combine the flour, ground pretzels, and sugar, mixing with your fingers to sift and combine. Add the melted butter and mix again until you have a soft but not wet dough.
3) Press the dough into the pan and bake for 10 to 12 minutes. The crust won't brown much, but will firm up nicely. Remove it from the oven and cool completely.
4) To make the filling: In a double boiler or in the microwave, melt the chocolate chips. Add the peanut butter, stirring until well combined and smooth. Pour the filling over the crust.
5) Spread the filling evenly over the crust, and tap the pan on the counter a few times to remove air bubbles. Sprinkle more crushed pretzels on top for added crunch.
6) Chill in the freezer for 45 to 60 minutes. Use the parchment to lift the bars out of the pan. Slice into squares or bars. You can serve frozen; just chilled, or at room temperature — these bars are great no matter what!
Yield: one 9 x 13" pan, about 40 servings.