For the choux pastry, sift the flour into a bowl. Place the butter and water into a saucepan and heat until the butter has melted.
Bring up to the boil, then remove from the heat and add all the flour in at once. Beat with a wooden spoon until the dough leaves the sides of the pan in a ball. Return to the heat and stir continuously for 2 minutes until the dough glistens. Tip out into a bowl and allow to cool a little.
Add the vanilla extract to the beaten eggs and beat into the dough a little at a time, reserving a tiny amount of the beaten egg for brushing later. The mixture should be very shiny and just fall from the spoon.
Preheat the oven to 220C/425F/Gas 7.
Line a baking sheet with non-stick baking paper. Scoop the dough into a piping bag and pipe a 15cm/6-inch circle on to the baking paper. Add a drop of water and a tiny pinch of salt to the remaining beaten egg and use to brush the top of the ring. Sprinkle generously with flaked almonds and place in the oven for 10 minutes.
Turn the heat down to 180C/350F/Gas 4 and continue cooking for a further 30 minutes. Remove from the oven and transfer to a wire rack. Immediately cut in half horizontally to release all the steam. If the inside is still soft and doughy, return to the oven for a few more minutes to dry out. Leave in two halves to cool thoroughly.
Now make the crème pâtissière. Beat the egg yolks and sugar in a bowl until pale and fluffy, then stir in the flour.
Bring the milk to the boil and add the coffee, salt and vanilla. Whisk the milk mixture into the egg mixture, return to the pan and whisk continuously over a moderate heat. If lumps start to form, simply remove from the heat and whisk hard until smooth again. Keep whisking until smooth and creamy, then cook for a further 2 minutes. Scrape out into a bowl and leave to cool.
To finish the gateau, place the bottom half of the ring on a serving plate, pipe on the crème pâtissière and set the upper ring on top. The cream should 'squidge' out of the sides. Dust generously with icing sugar and serve.
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