Which phase does the cytoplasm split to create two separate cells?

Cytokinesis!

Explanation:

Mitosis is divide into four phases: prophase, metaphase, anaphase, and telophase and the shortest phase is called cytokinesis. Cytokinesis is the division of the cytoplasm and it's organelles and it divided in to two daughter cells. Both daughter cells contain a nucleus with an identical set of chromosome.