In recent years, the bioconversion of organic waste using Hermetia illucens larvae, commonly known as Black Soldier Fly (BSF), has emerged as an innovative and environmentally sustainable solution. This study aimed to assess the efficiency of frass fertilizer produced by BSF larvae (BSFL) reared on a composite substrate consisting of rice husk biochar (RHB) and chicken manure (CM). A factorial randomized block design with two factors, and replications were employed. The first factor was the proportion of mixed substrates, consisting RHB and CM at different ratios (v/v): 10% biochar (B1), 30% biochar (B2), and 50% biochar (B3). The second factor was the feeding rate for BSFL: 200 mg per larva per day (M1) and 100 mg per larva per day (M2). Feeding was carried out for 16 days, after which the frass was sieved through a 2 mm mesh. The resulting frass was designated as Kasgotchar. The parameters observed included BSFL growth performance, waste reduction efficiency (WRE), and frass quality. The results revealed that the formulation of the RHB and CM substrate significantly influenced larval growth, frass weight, WRE, residual substrate percentage (%SR), and nutrient composition (pH, P, K, C/N ratio, and Ca content). Different substrate ratios significantly affected larval growth performance, WRE, and frass quality. The highest larval growth was observed in the B2 (30:70 RHB/CM) treatment combine with M1 (200 mg/day). The highest WRE was recorded in B3 (50:50), M2 (100 mg/day), and B2M2 (30:70 and 100 mg/day). The best frass quality was obtained from B1 (10:90), M2 (100 mg/day), and B1M1 (10:90 and 100 mg/day).