First, grab a large baking sheet and line it with parchment paper or a silicone baking mat. Set it aside – you'll need it ready.
In a medium saucepan, combine the butter, brown sugar, and light corn syrup. Pop it over medium heat.
Stir constantly as the butter melts and the sugar dissolves. Once it starts to bubble, bring it to a boil.
Continue boiling, stirring occasionally, for about 3 minutes. You'll see it darken slightly and thicken. Be careful, it's hot!
Remove the pan from the heat immediately. Quickly stir in the vanilla extract and the baking soda. It will get foamy – that's exactly what you want!
Pour the hot toffee mixture directly over the mini pretzels in a large heat-safe bowl. Gently fold them with a spoon or spatula until they are mostly coated. Work quickly before the toffee hardens!
Add the Heath toffee bits to the coated pretzels and fold again to distribute them evenly.
Carefully spread the pretzels out in a single layer onto your prepared baking sheet. Try to separate any large clumps so they can cool properly.
Let them cool completely at room temperature. This takes about 30-60 minutes. Don't rush this step – the toffee needs to harden to get that perfect crunch!
Once cool, break apart any pieces that are still stuck together. Your irresistible Butter Toffee Pretzels are ready to munch!