A prismatic-mesh periodic finite element boundary integral (P-FEBI) simulation tool is paired with a high performance real-valued evolutionary strategy to efficiently design frequency selective surfaces. The irregular, triangular meshing scheme is used to produce metallic structures of nearly any arbitrary shape and contour inside of the unit cell. The unit cell is bound by a parallelogram which permits a hexagonal unit cell grid, allowing the use of increased unit cell size before diffraction occurs compared to square or rectangular lattices. The relatively new covariance matrix adaptation evolutionary strategy (CMA-ES) is used as an effective tool to find the optimum configuration of metallic shapes and dielectric properties that yield the desired electromagnetic scattering and radiation characteristics. An antenna for use in a planar array and a frequency selective surface are created using the outlined method to illustrate the flexible design capabilities of the design technique.