We have developed a multilayer structure with a smooth surface and fabrication processes for HTS single-flux-quantum (SFQ) circuits. The multilayer structure with surface roughness R a less than 2 nm, composed of a La 0.2 -Y 0.9 Ba 1.9 Cu 3 O x ground plane and base electrode layer, and SrSnO 3 insulating layers, was deposited by off-axis magnetron sputtering. We have fabricated interface-engineered junctions based on the multilayer structure using an La 0.2 -Yb 0.9 Ba 1.9 Cu 3 O x counter-electrode layer prepared by pulsed laser deposition. The fabricated junctions exhibited excellent Josephson characteristics with a 1-σ spread in I c as low as 8% for 1000 junctions. The sheet inductance values below 50 K were 0.8-1.0 pH per square. Operation of several elementary SFQ circuits including a toggle flip-flop, a confluence buffer, a set-reset flip-flop and other SFQ elements has been successfully demonstrated at 30-60 K.