Set the oven to 200 °F, lightly coat a large roasting pan with cooking spray, and fill it with the pretzels. The pan should leave enough working room to turn the pretzels without spilling them over the sides.

Heat the brown sugar, cubed butter, and corn syrup together in a medium saucepan over medium heat, stirring continuously through a full 5-minute boil. The butter should disappear into a bubbling, cohesive syrup rather than remain in separate pieces.

Take the saucepan off the burner and whisk the vanilla and baking soda into the hot syrup. Expect the mixture to expand rapidly into a paler foam, and keep your hands clear of the rising glaze.

Distribute the hot glaze across the pretzels, add half the toffee bits, and gently fold everything together. Some pieces may look heavily coated while others look nearly bare, but the oven stirring will help even out the mixture.

Keep the pan in the oven for 1 hour, lifting and folding the pretzels every 15 minutes. Each turn should bring glaze from the bottom of the pan back onto the pretzels without crushing their loops.

Empty the baked mixture onto a wax paper-lined countertop, spread it out, and scatter the remaining toffee bits over it immediately. The warm coating will help the fresh bits cling as the clusters begin to set.

Leave the pretzels undisturbed for about 1 hour to cool and harden before breaking them into smaller clusters and packaging them. Ready clusters feel cool and firm rather than warm, flexible, or tacky.