Preheat the oven to 200°C. This is important because eclairs do not rise well in a cold oven.
Prepare the dough. Pour water into a saucepan, add sugar, salt and butter. Bring to a boil and simmer for 3-4 minutes. Add the sifted flour and stir the dough with a spatula until it begins to pull away from the sides of the pan. Remove from heat and let the dough cool slightly. Beat the eggs into the dough one at a time and mix well. The dough should have a creamy consistency.
Prepare the cream. Pour milk into a saucepan, add sugar and eggs. Add sifted flour and vanilla, stir well with a whisk. Place the pan on the fire and heat, stirring continuously, for 12-15 minutes. The cream should thicken and stick to the whisk or drain slowly from it. Let it cool for 10 minutes and add soft butter. Do this gradually, stirring constantly. Cover the cream with cling film so that it does not dry out.
Place the dough in a piping bag fitted with a star tip and pipe it into strips onto the baking sheet. Leave a gap of 3-4 cm between the cakes, as they will increase in volume. Bake at 200°C for 10 minutes, then reduce the temperature to 180°C and bake for another 30-35 minutes. Cool the cakes in the slightly open oven, otherwise they may settle due to a sudden temperature change.
Once the cakes have cooled, make small holes in the bottom and use a piping bag fitted with a tip to fill them with cream.
For the glaze, mix lemon juice and boiled water, gradually add powdered sugar in small portions until you reach the desired consistency and color. Spread frosting over cakes.
Serve classic eclairs with tea or coffee, decorating them with cream, chocolate, glaze or sprinkled with powdered sugar as you wish.