In a large bowl, mix (using whisk attachments) the butter with a hand-held or stand mixer on medium speed until fluffy. Add the powdered sugar one cup at a time until just combined. Add the lemon juice 1 tablespoon at a time until the desired consistency has been reached. Mix on medium speed for 3 minutes. Add the lemon zest and mix until just combined.
Refrigerate until ready to spread on baked goods.
Serving: frosts one double-layer cake or 24 cupcakes