Photograph: Kate Whitaker
Bring the milk to simmering point. Mix the yolks, sugar and flour in a bowl, then pour on the hot milk, whisking.
Return the custard mix to the pan; bring to a simmer over a low heat, stirring, until thickened. Tip the custard into a bowl, beat in half of the coffee, leave to cool, then chill.
Break the amaretti biscuits; divide among the glasses. Sprinkle with the Kahlúa and the remaining coffee. Top with equal amounts of coffee custard.
Whip the cream; top each trifle with a quarter of the cream and dust with cocoa. Finish each one with chocolate curls.
Get ahead: assemble the trifles up to a day ahead; keep chilled.