In a 2-quart saucepan, whisk together key lime juice, sugar and lavender. Whisk in egg and egg yolk.
Cook over medium-low heat, for about 1 minute, scraping down the sides often.
Remove from heat and stir in butter. Return to stove and cook over low heat for about 2 minutes, removing periodically and stir as it begins to thicken and bubbles start to form.
Pour finished curd into a bowl and cover with plastic wrap, pressing it against the curd so water doesn't form on top. Chill at least one hour before use.