In a mixing bowl with cookie paddles, cream together the butter, peanut butter, brown sugar and white sugar until smooth and creamy. Then add the eggs, cornstarch, vanilla, soda, powder and continue to mix until combined. Crush one cup of pretzels into small bits and pieces. Add the pretzels to the cookie mixture along with the flour. Add the salt and chocolate chips and continue to mix until all of the dry ingredients are combined. Grease a 9x13 pan with cooking spray and pour the batter into the pan. With a knife, try and get the batter as even as possible. Bake the bars at 350 for 20-25 minutes, or until the center of the bars are cooked through and no longer gooey (don't over bake though because the bars will set up as they rest on the counter). As soon as the bars are taken out of the oven, sprinkle on the top a few extra chocolate chips, some more crushed pretzels and drizzle with caramel topping (ice cream topping works great). Let bars rest at room temperature for an hour or longer, or until the bars set up. Cut into squares and serve. These bars can be eaten warm with vanilla ice cream which is also delicious! ?