In a small bowl, mix butter, lemon juice, capers, and flaked salt. Using a clean hand or spoon mix until well incorporated. Let sit for about 5 minutes in the extra lemon juice. Give it another mix, and drain any remaining juice.
Place butter mixture on a piece of parchment paper and roll into a log shape. Place it in the freezer until ready to use. To make ahead, chill in the refrigerator overnight or until ready to use.