Methods for manufacturing airfoils of gas turbine engines are provided. The methods include forming a main body core, the main body core including a feed cavity core portion, the main body core configured to form at least a part of an airfoil including an airfoil body, a platform, and an attachment element, forming a platform circuit core having a platform core extension, wherein the platform circuit core is configured to form a cooling circuit in the platform, wherein at least one of the feed cavity core portion and the platform core extension includes a notch, assembling the platform circuit core to the main body core such that the platform core extension engages with the feed cavity core portion at the notch, casting an airfoil using the assembled platform circuit core and main body core, and removing a platform core extension element that is formed at the location of the notch.