Step 1: Preheat oven to 300°F (150°C). Line a large baking sheet with parchment paper or a silicone baking mat.
Step 2: Spread the mini pretzels evenly in a single layer on the prepared baking sheet.
Step 3: In a medium saucepan, combine the butter and brown sugar. Heat over medium heat, stirring constantly, until the butter is melted and the sugar is dissolved.
Step 4: Bring the mixture to a rolling boil and continue to simmer for 3-5 minutes, stirring occasionally. The mixture should thicken slightly and turn a rich caramel color.
Step 5: Carefully pour the hot toffee mixture evenly over the pretzels on the baking sheet. Use a heat-proof spatula to gently toss the pretzels, ensuring they are all coated.
Step 6: Bake for 15-20 minutes, stirring gently halfway through to redistribute the pretzels and prevent burning. The toffee should be bubbly and the pretzels lightly golden.
Step 7: Remove from the oven and spread the pretzels out on the baking sheet to prevent clumping. Let them cool completely at room temperature, undisturbed, for at least 1-2 hours until the toffee hardens. Once cooled, break any larger clusters into individual pieces. Store in an airtight container.