Mathematical spaces often reinforce and perpetuate whiteness. Despite efforts to decenter whiteness in secondary and post-secondary classrooms, equity-oriented strategies can still perpetuate whiteness. In this action research study, I turned the lens of whiteness upon my instructional practices and curriculum. Using an analytic framework for whiteness in mathematics education with three dimensions (i.e., "institutional," "labor," "identity"), I explored how my design and delivery of a social justice mathematics course perpetuated whiteness. I identified prominent themes related to how I maintained white supremacy culture, specifically through the "institutional" and "labor" dimensions, by maintaining power and authority in the course. In addition to recognizing areas where I upheld whiteness, I outlined my ongoing efforts to decenter whiteness through engaging students in ethnomathematics investigations, co-constructing curricular resources, and repositioning course stakeholders. This action research study informs ongoing efforts to reimagine mathematics and mathematics education as white institutional spaces by critically interrogating my mathematics classroom practices.