Physical computing has emerged as an effective approach to introducing computing and coding to students. One of the most popular enabling tools is the BBC micro:bit, well-known for its positive impact on teaching programming and driving engagement in the classroom. We extend these benefits by developing a new approach to coding with micro:bit: MicroCode. Unlike other experiences, MicroCode couples the micro:bit with a low-cost handheld accessory to enable live and portable programming via an on-device visual programming language; no separate host computer is needed. We present the design of MicroCode and the findings of a study in which we interviewed five primary school teachers and 60 children aged 10-11 working with MicroCode. The outcomes of the study show that MicroCode raised children's engagement and stimulated the development of a strong sense of agency on coding activities, while teachers felt empowered to adopt situated and cross-curricular learning approaches.