In a medium-sized, heavy-based pot, bring the butter, sugar and vanilla extract to a boil. Keep stirring until the caramel starts to turn golden-brown (7–10 minutes) and reaches 155 °C on a sugar thermometer, then pour it into the prepared dish or baking tray and spread it out evenly.
Quickly arrange the pretzels evenly across the surface of the toffee (making sure you don't touch the hot caramel and press down lightly.
Leave it to cool for a few hours or overnight.
Melt the chocolate on the stove in a double-boiler or in a microwave, and spread it evenly over the caramel. Allow to cool and harden
When the toffee and chocolate have cooled completely, break them up into shards or cut them into about 24 pieces. A few bits will break off.