AI systems are widely used for automation, productivity, and information generation, yet they also have the potential to function as active partners in human cognition. This paper introduces “Cognitive AI,” a theoretical framework that positions AI as a catalyst for intentional neuroplasticity capable of expanding human cognition. Drawing on research in distributed cognition, adult development, and the science of neuroplasticity, the framework describes how iterative interaction with AI can serve as a catalyst to trigger intentional cognitive practices that can reorganize neural patterns and expand human cognition. The implementation of an “AI Second Brain” pilot illustrates how the framework can appear in lived cognitive processes, showing that real-time collaboration with AI can activate the proposed developmental dynamics. These insights also suggest that related effects may emerge in the future across other forms of intelligent systems, including embodied agents.