First make the mousseline. Place the milk in a saucepan and bring to the boil. While the milk is heating, place the egg yolks and sugar in a large bowl and beat with electric whisk until pale and fluffy. Then add the flour and mix again to incorporate.
Once the milk has reached boiling point, strain half the milk into the egg mixture, whisking until smooth. Add the remaining milk and mix, then pour the mixture back into the pan, whisking all the time and return to the heat. Bring to the boil and cook gently for 1-2 minutes, still whisking. Pour into the bowl and add half the unsalted butter, stirring to melt. Leave to cool.
Once cooled, using an electric mixer, beat the remaining softened butter into the mousseline mixture, then beat in the hazelnut paste. Chill for 30 minutes in the fridge.
Now make the choux pastry. Preheat the oven to 185°C/gas mark Line two baking trays with baking paper.
Heat the milk and water then add the butter and once melted bring to a fast boil. Remove from heat and immediately add all the flour. Return to the heat and beat vigorously with a wooden spoon until the mixture leaves the sides of the pan and is smooth and glossy.
Allow the mixture to cool a little, then add the eggs, a little at a time, beating well after each addition. You will be left with a smooth and glossy mixture.
Spoon the mixture into piping bag and pipe 8 small rings, about 10cm in diameter onto lined tray. Then pipe a second ring on top of the each first ring. Glaze with a beaten egg and sprinkle with flaked almonds.
When ready to bake, turn the oven down to 180°C and place the choux pastries in the oven and cook for 30 minutes until golden brown, watching them closely. Remove from the oven and cool on a wire rack.
To assemble, once cool, place a choux pastry ring on plate and cut in half horizontally. Spoon the mousseline into a piping bag and pipe over the bottom half of each choux ring. Place the other half on top and dust with icing sugar to serve.