Preheat oven to 250°F. Line two large baking sheets with parchment paper. Set aside.
Spray a large bowl with cooking spray. Place pretzels in it. Set aside.
To a large saucepan over medium heat, add brown sugar, butter, corn syrup, salt, and vanilla. Bring the mixture to a boil, stirring occasionally. This can take up to 8-10 minutes, but watch it carefully. Once boiling, immediately remove from heat. (If using a candy thermometer, the mixture should reach a temperature of 234°F.) *
Whisk in the baking soda (the mixture will bubble up and become 'foamy'), continuing to whisk until the baking soda is dissolved.
Pour the toffee mixture over the pretzels. Gently stir until all pretzels are coated.
Pour the coated pretzels evenly onto the two lined baking sheets, spreading them out into a single layer.
Bake for 60 minutes, gently stirring the pretzels every 10-15 minutes.
Remove the pretzels from the oven. Let them cool to room temperature before serving, stirring occasionally to prevent clumping. Enjoy!