Preheat oven: to 180°C / 350-375°F / Gas Mark 4-5.
Baking tray: line a 20 x 30 cm (12 x 8 inches) baking tray with 2 overlapping sheets of parchment paper with overhang on the edges.
Butter and sugar: add the butter and sugar to a heavy based sauce pan. Using medium to low heat allow the butter to melt and sugar to soften. Let these fully combine (will be consistent pale brown colour) and once they have let them boil and bubble away without stirring for 30 seconds.
Toffee and pretzels: once the toffee has boiled add the pretzels to the saucepan. Stir to combine and cover them in the liquid. Once done pour this mixture into the prepared baking dish and use a spatula to spread it out into one (evenish) layer.
Oven Bake: next place the baking tray into the preheated oven (middle shelf, uncovered) and let the pretzel crack bake for 8 minutes.
Chocolate: in the final 3 minutes of cook time, place the chocolate into the microwave (in a heatproof jug). Microwave on high for 2 minutes then remove, set on a heatproof surface and stir. Return to the microwave for 1 further minute on high then repeat the remove and stir step. Use oven mitts to remove the baked pretzels from the oven and set the tray on a heatproof surface. Drizzle the melted chocolate over the top and then use a spatula to spread out over the baked pretzel layer.
To decorate: press the reserved pretzels and pieces into the melted chocolate to cover the surface (spread out a little). Then sprinkle sea salt over the top.
Set: place the decorated Christmas pretzel crack into the fridge and let set for 2 hours until firm to the touch.
To serve: remove the set pretzel crack from the fridge. Use the overhang to lift it up and out of the baking tray. Discard the paper. Break the pretzel crack into pieces using your hands (or sharp knife and cutting board). Stack into bowls or a large platter and serve.