Firstly, preheat the oven to 190C/170C/Gas Mark 5 and line a large baking tray with baking paper.
In a large bowl, cream the butter and sugar together. Then add the flour and mix until it resembles a rough lumpy dough. Then, finely chop the chocolate and pretzels and mix in to the dough. It may be easier to squeeze the dough together with your hands to get the final chunks of chocolate and pretzel mixed in.
Generously flour your worksurface and roll the dough out until 1cm thick. Then using a 6cm cookie cutter, cut out 16 rounds. Try not to re-roll the dough too many times while doing this. Place the cookie rounds onto your prepared baking tray, and place in the fridge to chill for 20 minutes. Then sprinkle each cookie dough generously with caster sugar, then place in the oven to bake for 12-14 minutes until lightly golden brown.
Once baked, sprinkle with a little more sugar and place on a cooling rack to cool completely.
Notes
Cookies will keep for 3-4 days in an air-tight container.