Endearingly is the adverb: you do something in a way that makes people love or like you. She smiled endearingly. He endearingly mixed up everyone’s names and then apologised twice. It describes the manner of the action, not the person as a whole. The feeling is the same soft fondness as endearing. Though actually, writers sometimes use it with a tiny smile in the voice — “endearingly wrong” — meaning the mistake is so human that you like them more, not less. It is a little literary. In speech, people often just say “in a sweet way,” or they use the adjective instead.