Hanoi takes its coffee seriously, and in the last fifteen years coconut coffee has become one of the city's signatures, sold from Old Quarter cafés, lakeside balconies around Hoàn Kiếm and tiny stools on the pavement. It is a hot-weather drink, at its best on a sticky afternoon between May and September, eaten half with a spoon. Strong Vietnamese robusta drips slowly through an aluminium phin, then is poured over a pale slush of thick coconut cream, sweetened condensed milk and crushed ice, so you get bitter, dark coffee against something rich and cold that tastes like coconut ice cream. The common mistake is blending the slush too long or with too much ice, which leaves it thin and watery, and pouring the coffee in boiling hot so the slush collapses. Blend just until it looks like soft snow, and let the coffee cool for a minute or two. It is ready when the slush holds a spoon upright and the coffee sits in a dark layer on top. Robusta is high in caffeine and this is a strong drink, so it is not for children and one glass is plenty. Chef's tip: chill the coconut cream overnight; cold, thick cream gives a fluffier slush.