Pulse together the bread mix and sugar in a food processor or table-top mixer. While the motor is running, add the egg and about 250-300ml lukewarm water, until a soft dough is formed. Knead on a lightly floured surface until smooth, about 5-10 mins. Leave in an oiled bowl, covered with oiled cling film, in a warm place until doubled in size - about 1 hr.
Knock back dough by squashing with your fist, and divide into 20 even-sized pieces - keep covered with a clean tea towel so they don’t dry out. Shape each into a sausage and place on an oiled baking tray. Cover with oiled cling film and rise until doubled in size again.
Heat oven to 200C/180C fan/gas 6. Remove film and cook buns on top shelf for 8-10 mins, until golden. Cool on a wire rack. To decorate, mix icing sugar with a little water until stiff but spreadable - add food colouring, if you like. Dip in the top of each bun and scatter with sprinkles.