Put the butter in a large saucepan with 225ml water, heating slowly so that the butter melts but the water doesn’t evaporate. Remove from the heat and quickly add the flour. Mix until fully incorporated and comes away from the pan, then leave to cool to room temperature. Use a wooden spoon to beat in the eggs, one at a time, until smooth, glossy and the mix drips off the spoon after 3 secs. Beat in the cheese until combined, before folding in the chives, crab and some seasoning.