Heat some olive oil in a saucepan over medium-high heat, add herbs, fry until crisp, strain trough a heatproof sieve to discard the oil. Transfer to a mortar and pestle. Add salt and pound until coarsely ground, then set aside.
Beat butter in an electric mixer until pale and fluffy (4-5 minutes). Pipe onto radishes using a piping bag. Serve with the herb salt.