A rotary cutting tool includes a replaceable cutting insert, a pocket portion for holding the cutting insert, and a chip flute portion having a plurality of helical flutes. An internal coolant passage is formed in the chip flute portion and has both a twisted geometry and a “see-through” central portion. The internal coolant passage is polygonal in cross-sectional shape having a number of cusps equal to the number of flutes. The internal coolant passage greatly increases coolant flow area, while maintaining the stiffness and rigidity of the cutting tool. A method includes forming the internal coolant passage using additive manufacturing such that the internal coolant passage has a “see-through” central portion and a twisted geometry.