Special equipment: a piping bag fitted with a 1-centimeter (0.4-inch) plain tip
Heat your oven to 190 degrees C (375 degrees F). Line a large baking tray with parchment.
For the choux pastry: Put the milk, water, butter, sugar and salt into a large saucepan. Heat gently until all the butter has melted then bring to a boil. Add the flour. Beat with a wooden spoon to form a smooth ball of dough that leaves the sides of the pan, 2 to 3 minutes.
Transfer the dough to a large bowl. Now vigorously beat the eggs one at a time into the dough. This takes a bit of elbow grease. The dough will become glossy. Stop adding the egg if the dough starts to become loose.
Transfer the mixture to a piping bag fitted with a 1-centimeter (0.4-inch) plain nozzle. Pipe into an Eiffel Tower or other shape. Bake until risen, crisp and golden, 30 to 35 minutes. Remove from the oven and cool on a wire rack.
When cooled, decorate with the melted dark and white chocolates. Serve with whipped cream on the side.