Line a baking sheet with waxed or parchment paper.
Add butter and brown sugar to a small saucepan. Bring to a boil over medium-high heat. Continue to cook, stirring occasionally, until a candy thermometer reaches 300 degrees. If the mixture is getting too brown, turn down the heat so it doesn't burn.
Scoop mixture onto the lined baking sheet and allow to cool.
Once cooled, break up the toffee into pieces and set aside. You can double-wrap them in a plastic bag, or just place another paper on top before crumbling them with a mallet.