Preheat oven to 200°F and spray a large roasting pan with cooking spray. Add pretzels.

Combine brown sugar, butter, and corn syrup in a saucepan. Bring to a boil over medium heat, stirring constantly.

Let mixture boil for 5 minutes while stirring.

Remove from heat and whisk in vanilla and baking soda.

Pour hot toffee mixture over pretzels.

Sprinkle half the toffee bits and toss to coat evenly.

Bake for 1 hour, stirring every 15 minutes.

Spread pretzels onto wax paper while warm and sprinkle remaining toffee bits.

Let cool for 1 hour, then break into clusters and store.