Start by melting one third of your butter and cutting the rest of the butter into small-ish cubes.
Add your melted butter to a food processor (or a blender, or a bowl if using a regular mixer) together with a few of the cubes of solid butter. Start mixing, then add more butter cubes until they have all been added.
Add your lemon juice, then crush the sea salt flakes in your hand and add them as well. Mix well and taste test. Mix more and adjust seasoning if needed. Serve!