Preheat your oven to 200°F (about 93°C). Line a baking sheet with heavy-duty aluminum foil and generously spray with nonstick cooking spray.

Pour the pretzels into an extra-large heat-safe bowl.

In a 3- to 4-quart saucepan over medium heat, add the brown sugar, sliced butter, and light corn syrup. Stir constantly and bring the mixture to a low boil. Continue boiling for about 5 minutes.

Remove the saucepan from the heat and carefully stir in the vanilla extract and baking soda. Be prepared: the mixture will bubble up and thicken quickly.

Immediately pour the hot toffee mixture over the pretzels in the bowl. Use a wooden spoon or heat-safe silicone spatula to stir until the pretzels are completely coated.

Transfer the coated pretzels to the prepared baking sheet, spreading them out in an even layer. Sprinkle half the toffee bits (4 ounces) over the coated pretzels.

Bake in the preheated oven for 1 hour, stirring (or turning) the toffee pretzels every 15 minutes to help ensure even coating.

As soon as you remove them from the oven, sprinkle the remaining toffee bits over the warm pretzels. Allow them to cool completely before breaking apart into clusters.