A method and system for fabricating a composite structure is disclosed. A composite preform (10) has an upper surface (14) and an opposing lower surface (15). The upper and lower surfaces (14, 15) each define a preform major surface. A heat sink (120) is located in proximity to one of the preform major surfaces so as to extend across only a portion (16) of the composite preform (10). A resin is cured in the composite preform (10) to form the composite structure. The resin cures exothermically. During curing of the resin, heat is conducted away from the portion (16) of the composite preform (10) into the heat sink (120).