Allow the butter to come to room temperature before starting. Beat the butter until creamy, then add in the parsley, red pepper flakes, black pepper, minced garlic, lemon juice and zest. Mix until well combined.
1 cup salted butter, 1/4 cup fresh parsley, finely minced, pinch red pepper flake, pinch coarsely ground black pepper, 1 clove garlic, finely minced, 1/2 lemon juiced and zested