Look at these cases of Adverbs in the Superlative:
Note that to form the Superlative of adverbs, it is similar to the Superlative of adjectives, EXCEPT you only use the definite article le plus / le moins (there is NO agreement with adverbs, as they have NO gender).
You NEVER say ''Jeanne parle la plus doucemente.''
Here are other Superlative structures:
Le, la, les plus and le, la, les moins = the most and the least (superlatives of adjectives)
Meilleur, mieux, pire / plus mauvais, plus mal = better, best, worse and worst (irregular comparatives and superlatives)
And Comparative structures:
Plus... plus..., moins... moins... = the more...the more..., the less...the less... (comparisons with phrases)
Better and better, worse and worse = de mieux en mieux, de pire en pire (comparisons)
De plus en plus and de moins en moins = more and more and less and less (comparisons with adjectives, adverbs, verbs)
De plus en plus de and de moins en moins de = more and more and less and less (comparisons of nouns)
Making comparisons with adjectives: plus... que, aussi... que, moins... que
Making comparisons with adverbs: plus... que, aussi... que, moins... que
Making comparisons with verbs: plus que, autant que, moins que
Making comparisons with nouns: plus de... que, moins de... que, autant de... que