For the cleanest swirl, keep the raspberry preserves and ganache slightly thick so they sit in the batter rather than sinking; if the ganache gets too firm, rewarm it for a few seconds until pourable. Use room-temperature ingredients to ensure a smooth batter and even rise, and avoid overmixing once dry ingredients are added to keep the crumb tender. If you prefer less sweetness, use a tart seedless raspberry jam or reduce the ganache drizzle slightly. Check cake doneness a couple of minutes early, as red velvet can overbake quickly and dry out. The cake layers and ganache can be made a day ahead; wrap layers tightly and refrigerate, and gently rewarm ganache before assembly.