An improved cast booster design and method of assembly is provided for the detonation of blasting agents. The booster design utilizes a pre-formed core that simplifies assembly of the booster and increases the reliability of detonation transfer from a blasting cap. The pre-formed core has a cup shaped aperture provides improved coupling with the initiation source. The pre-formed core is made using a relatively insensitive explosive composition that can be manufactured using high speed pressing methods. The explosive composition allows for the attainment of a well defined shape with a predictable density. The pre-formed core shape mates with the casting mold in a way that ensures the location of the core within the booster thereby improving reliability and reducing labor associated with the booster manufacturing operation.