Line a 9x13-inch baking dish with parchment paper and layer the bottom with mini pretzels.
In a saucepan over medium heat, melt the butter and brown sugar together. Bring to a gentle boil and stir constantly for about 3-5 minutes until it thickens slightly.
Pour the caramel mixture evenly over the pretzels and spread to cover.
In a microwave-safe bowl, melt the caramel bits with milk in 30-second intervals, stirring until smooth. Pour over the caramel layer.
Sprinkle the semi-sweet chocolate chips on top. Allow them to melt slightly and then spread evenly.
Optional: Melt white chocolate chips and drizzle over the top for decoration.
Refrigerate until set, about 1-2 hours. Once firm, cut into bars and enjoy!