Shell and blanch the chestnuts. Put them in a saucepan and almost cover them with boiling water, cook until tender. Before they are quite done add a little salt. When done remove from the fire, rub through a purée sieve with the water they were boiled in. Melt the butter with the flour and add to it by degrees the boiling milk, let it cook until thick, then stir in the chestnut purée and salt and pepper to taste. Let it come to a boil and serve.