Here, for the first time, we demonstrate a facile one-step construction of a nitrogen-doped porous carbon building (N-PCB) using interconnected ultra-small carbon nanosheets through the carbonization of biomass (Auricularia) using ZnCl₂ as the activating agent and NH₄Cl as the nitrogen source. Due to its high specific surface area (1607 m² g⁻¹) with a high mesopore ratio (91%), interconnected porous structure with short ion diffusion paths, and nitrogen doping (4.8 at%), the N-PCB electrode exhibits a high specific capacitance of 347 F g⁻¹ at 1 A g⁻¹, excellent rate capability (278 F g⁻¹ at 50 A g⁻¹, 80% of capacitance retention) and outstanding cycling stability (only 2% loss in specific capacitance after 10 000 cycles). Moreover, the assembled N-PCB symmetric supercapacitor is stabilized with excellence cycling stability (4% loss after 20 000 cycles) at 1.6 V in 1 M Na₂SO₄ aqueous electrolyte, delivering a high energy density of 22 W h kg⁻¹, much higher than that of most of the reported carbon-based symmetric supercapacitors in aqueous electrolytes. Therefore, these exciting results suggest a low-cost and environmentally friendly design of electrode materials for high-performance supercapacitors.