Breast cancer is a deadly disease that affects countless women worldwide. The most conventional treatments for breast cancer, such as the administration of anticancer medications such as letrozole (LTZ), pose significant barriers due to the non-selective delivery and low bioavailability of cytotoxic drugs leading to serious adverse effects and multidrug resistance (MDR). Addressing these obstacles requires an innovative approach, and we propose a combined strategy that synergistically incorporates LTZ with berberine (BBR) into stabilised AuNPs coated with ascorbic acid (AA), known as LTZ-BBR@AA-AuNPs. The LTZ-BBR@AA-AuNPs, a novel combined drug delivery system, were carefully designed to maximise the entrapment of both LTZ and BBR. The resulting spherical nanoparticles exhibited remarkable efficiency in trapping these two compounds, with rates of 58% and 54%, respectively. In particular, the average hydrodynamic diameter of these nanoparticles was determined to be 81.23 ± 4.0 nm with a PDI value of only 0.286, indicating excellent uniformity between them. Furthermore, their zeta potential was observed to be −14.5 mV, suggesting high stability even under physiological conditions. The release profiles showed that after being incubated for about 24 h at pH levels ranging from acidic (pH = 5) to basic (pH = 7), the percentage released for both drugs ranged from 56–72%. This sustained and controlled drug release can reduce any negative side effects while improving therapeutic efficacy when administered directly to cancer. MDA-MB-231 cells treated with LTZ-BBR@AA-AuNPs for 48 h exhibited IC50 values of 2.04 ± 0.011 μg/mL, indicating potent cytotoxicity against cells. Furthermore, the nanoparticles demonstrated excellent stability throughout the duration of the treatment.