This study aims to examine the implementation of core banking systems (CBSs) in Islamic banking windows (IBWs) in Bangladesh, where conventional and Islamic banking coexist. It explores how IBWs balance operational efficiency with Shariah compliance when choosing between a standalone Islamic banking CBS (IB-CBS) and an Islamic banking module (IB-Module) within a conventional banking CBS (CB-CBS). The research analyses the technological, operational and governance aspects of CBS adoption. This study uses a qualitative research design that integrates survey responses from Islamic banking professionals with data on CBS adoption across IBWs in Bangladesh, further supported by semi-structured interviews with senior IBW executives. This triangulated approach enhances the credibility of the findings and ensures that operational realities are accurately captured. The thematic analysis focuses on Shariah compliance, operational efficiency, system integration and governance oversight. Despite practitioners' preference for IB-CBSs for Shariah compliance and flexibility in product development and innovation, most IBWs adopt IB-Modules due to institutional priorities, vendor dynamics and cost considerations. Middleware and application programming interface (API) driven integration is identified as potential solutions for reconciling efficiency with compliance in the implementation of IB-CBSs. The study's limitations include a small sample size, reflecting limited access to senior IBW executives and its focus on Bangladesh, which may limit generalisability across different jurisdictions. Nonetheless, the triangulated methodology enhances validity and provides a foundation for future comparative research. To the best of the authors' knowledge, this study is the first to systematically examine the adoption of CBS in IBWs. By integrating practitioner insights, it identifies how CBS design influences both operational efficiency and Shariah compliance in dual-banking contexts, while also highlighting limitations in current systems. By focusing on the "engine room" of IBWs - the CBS - the study addresses a critical gap and advances the understanding of Islamic banking infrastructure, providing insights for regulators, practitioners and policymakers on strengthening compliance, efficiency and market credibility in Islamic banking.