The paper presents a novel approach to fabricate ultra-stable, phase-pure formamidinium lead iodide (FAPbI3) films using two-dimensional (2D) perovskites with FA as the cage cation. By lattice templating, the 2D perovskites with FA cations are used to template the formation of FAPbI3, which forms a black phase at a significantly lower temperature (100°C) compared to the standard annealing temperature of 150°C. The resulting FAPbI3 films exhibit an efficiency of 24.1% in a p-i-n architecture with a 0.5 cm² active area and exceptional durability, with a T97 of 1000 hours under 85°C and maximum power point tracking (MPPT). The study demonstrates that the 2D perovskites with FA cations can nucleate first due to their lower enthalpy of formation and stability at room temperature, allowing the 3D perovskite to adopt the underlying 2D lattice periodicity during film annealing. This method provides a novel design strategy for the templated growth of 3D perovskites using 2D perovskites, offering improved stability and performance in optoelectronic devices.The paper presents a novel approach to fabricate ultra-stable, phase-pure formamidinium lead iodide (FAPbI3) films using two-dimensional (2D) perovskites with FA as the cage cation. By lattice templating, the 2D perovskites with FA cations are used to template the formation of FAPbI3, which forms a black phase at a significantly lower temperature (100°C) compared to the standard annealing temperature of 150°C. The resulting FAPbI3 films exhibit an efficiency of 24.1% in a p-i-n architecture with a 0.5 cm² active area and exceptional durability, with a T97 of 1000 hours under 85°C and maximum power point tracking (MPPT). The study demonstrates that the 2D perovskites with FA cations can nucleate first due to their lower enthalpy of formation and stability at room temperature, allowing the 3D perovskite to adopt the underlying 2D lattice periodicity during film annealing. This method provides a novel design strategy for the templated growth of 3D perovskites using 2D perovskites, offering improved stability and performance in optoelectronic devices.