Heat milk in a saucepan over low-medium heat to 80C (4-5 minutes). Add parmesan, stir well to combine, then set aside to infuse (40-50 minutes). Strain through a coarse sieve, then through a coffee filter into a jug, pressing lightly to extract all the liquid; you may need to replace the filters a few times as they become blocked (discard solids). Combine remaining ingredients, except shiso and oil, with 100ml cold water and ½ tsp sea salt flakes, then stir into infused milk. Pass again through a fine sieve and divide among four 160ml bowls. Steam in a large steamer over barely simmering water or in a steamer oven at 85C until just set with a slight wobble in centre (14-16 minutes). Set aside at room temperature.
For rosemary dashi, heat konbu and 250ml water in a saucepan over low-medium heat to 60C and maintain heat for 1 hour to infuse. Discard konbu and bring stock to 85C. Add bonito and rosemary, turn off heat and infuse briefly (10 seconds), then strain through a sieve lined with a double layer of muslin (discard solids) and set aside to cool (30-40 minutes). Add soy sauce, mirin and xanthan gum, blend with a stick blender to dissolve gum, then strain through a very fine sieve.
To serve, spoon a little rosemary dashi over chawanmushi (you may have some leftover; see note), scatter with shiso leaves and dot with oil.
