Of everything on the cha chaan teng breakfast board, this is the one to order. Ham macaroni soup at 320 calories beats satay beef noodles (667), luncheon meat and egg instant noodles (785) and a buttered toast with fried egg (427). The macaroni is boiled, not fried, and the ham is lean. The broth is mostly water with a bit of chicken stock powder, around 40 calories a bowl.
The set is the trap. The macaroni bowl comes as a breakfast set with two slices of buttered toast (about 250 calories) and a hot milk tea (110). That takes a 320 calorie bowl to a 680 calorie breakfast. Ask for the toast without butter and it drops by about 100, or skip the toast and the whole thing stays under 450 with the tea.
Watch the protein swap. Ham gives you about 14g of protein for 50 calories. Change it to luncheon meat (餐肉) and the same two slices are 190 calories with double the fat. Change it to a fried egg on top (煎蛋) and add another 110. The 餐蛋通粉 version, with both, is closer to 600 calories and is one of the most popular orders in Hong Kong.