Heat 1 cup (250 ml) water to boiling, take off the stove and pour the butter and salt in it while stirring. Add the flour all at once and stir to a smooth, homogeneous mixture.
Put the mixture to heat up again slightly, then pour it into another container.
Knead a smooth, shiny dough, put it in a pastry bag and spray out eclairs in a buttered and floured oven dish.
Bake the eclairs in a preheated 392°F (200 °C) oven.
For the cream mixture, beat 4 yolks with the sugar until whitened and add the cream. Dissolve the gelatin in 1/2 cup water in a water bath and add it to the mixture, while pouring in the beaten to snow egg whites at the end.
Cut the baked eclairs in 2 lengthwise and fill them with the prepared cream mixture after they cool. Optionally, you can pour melted couverture chocolate over them.