dataset.scala

def topHashtags(tweets: Dataset[Tweet], n: Int): Dataset[(String, BigInt)] =
  tweets
    .select($"text".as[String]) // select the text column (Dataframe)
    .flatMap(_.split("\\s+"))   // split it into words    (Dataset)
    .filter(_.startsWith("#"))  // filter hashtag words   (Dataset)
    .map(_.toLowerCase)         // normalize hashtags     (Dataset)
    .groupBy($"value")          // group by each hashtag  (Dataframe)
    .agg(count("*") as "count") // aggregate the count    (Dataframe)
    .orderBy($"count" desc)     // order                  (Datafeame)
    .limit(n)                   // limit to top results   (Dataframe) 
    .as[(String, BigInt)]       // set the type again     (Dataset)

Line 9: =~ s/DataFeame/DataFrame/