Everyday digital technologies are designed to maximise speed, ease of use, and continuous engagement. While these qualities support traditional usability goals, they can also undermine the cognitive capacities they were designed to support. This thesis introduces cognitive resilience as a complementary HCI design construct: the capacity to maintain core cognitive functions under the immediate and cumulative demands of digital interaction. Grounded in a review of the literature on digital technology's impact on cognitive functions, three design studies were conducted across distinct interaction contexts. Attention Mode introduces a cross-platform distraction-reducing feature, returning user control over attention during everyday media consumption. SmartPause adds breakpoints into digital reading, combining structured pauses with brief note-taking prompts; a between-subjects study (N = 51) and a within-subjects study (N = 14) showed gains primarily in memory retention. ReasonScape proposes a redesign of AI chat to guide reflection, reduce premature acceptance of AI outputs, and support reasoning in AI-assisted judgement tasks. Together, the studies follow a Research-through-Design methodology combined with mixed methods. Synthesis across the three studies produced a working definition of cognitive resilience for HCI, alongside design artefacts and a set of principles and guidelines. Together, these offer practical guidance for designers seeking to sustain users' cognitive engagement. By extending existing usability and UX discussions, this research argues that digital systems should be evaluated not only by how easily they enable task completion, but by how well they sustain users' attention, memory, and reasoning during and after the interaction.