Whisk the eggs and sugar in a heatproof bowl set over a saucepan of gently simmering water for 5 minutes until very thick and the whisk leaves a trail when lifted above the mixture. Remove from the heat then fold in the flour, butter and coffee.
Transfer the mixture to an oiled and base-lined 23 cm (9 inch) cake tin and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 25–30 minutes. Leave to cool in the tin for 5 minutes then turn out on to a wire rack. Slice the cake horizontally into 3 layers.
Put the nuts on a baking sheet. Heat the sugar and water in a heavy-based saucepan until the sugar dissolves. Increase the heat until the sugar turns light golden. Remove from the heat and pour over the nuts. Once set, break the praline into small pieces and then grind to a rough powder.
Beat the egg yolks and sugar together until pale. Heat the milk until just boiling, then whisk into the egg mixture. Return to the pan and heat gently, stirring, until the mixture coats the back of the spoon. Beat the mixture off the heat for 2–3 minutes and then gradually beat in the butter, a little at a time, until the mixture is thick and glossy. Beat in the maple syrup.
Fold half the praline into half the icing and use to sandwich the layers together. Spread the remaining icing over the top and sides of the cake and sprinkle with the reserved praline.