Preheat the oven to 350 degrees.
Beat the butter, peanut butter, granulated sugar, brown sugar, vanilla and egg with either a hand held mixer or a stand mixer until it's fluffy, about 2 minutes.
In a separate bowl, combine the flour, baking powder and salt and mix well.
Combine the wet and dry ingredients and mix to combine. Be careful not to over mix as it will make the cookie bars tougher. It's ok if there are a few streaks of flour in the batter.
Line an 8"x8" metal pan with parchment paper. Pour the dough in the lined pan and pat down in an even layer.
Firmly press the pretzels into the top of the cookie dough. Make sure they are set in good, otherwise the pretzels will fall off when you eat them.
Bake for 20-25 minutes (until edges are golden brown), remove from oven and allow to cool fully. Once cooled, slice the cookies into squares with a warm, sharp knife.
Melt the chocolate according to the instructions. Using a spoon, drizzle the chocolate over the cookie bars. You can use as little or as much as you'd like.