The DLC adds a massive amount of content to the already dense base game: A New Island to Explore:

The second part of the keyword is . RAR (Roshal ARchive) is a proprietary archive file format developed by Eugene Roshal. It is similar to a .ZIP file but often provides better compression ratios.

Here are some specific tips for defeating the new bosses:

The Delicious Last Course DLC takes place in a new area of the Cuphead world, called "The Cookie Kingdom." This mystical realm is ruled by the benevolent King, who is famous for his scrumptious cookies. However, a new threat emerges in the form of Chef Saltbaker, who seeks to take over the kingdom and monopolize the cookie-making business. Cuphead, Mugman, and Ms. Chalice must join forces to stop Chef Saltbaker and his minions, and save the Cookie Kingdom from certain doom.

In the end, Cuphead and Mugman emerged victorious, their bellies full and their hearts content. They realized that sometimes, the most delicious things in life are the memories you make with friends, and the experiences that come with trying something new.