In a medium bowl beat together the butter and sugar until fluffy.
Beat in the peanut butter, egg and vanilla extract.
Slowly beat in the flour and baking soda.
Turn off the mixer and stir in the chopped pretzels and chocolate chips.
Form the dough into balls about 1.5 tablespoons in size, then gently press a pretzel into the top of each dough ball.
Place on a tray, cover in cling film, and place in the fridge for at least 1 hour or up to 48 hours.
When ready to bake, preheat the oven to 350F degrees.
Take the cookie dough balls out of the fridge and place 2 inches apart on a cookie tray lined with parchment paper or a silicone baking mat.
Bake for 9-11 minutes, or until the tops look just set.