Making the choux pastry: In a saucepan, combine the water, salt, sugar and cubed butter. Bring to a boil to melt the butter. Remove from heat and stir in the flour and cocoa. Return the saucepan to medium heat and vigorously beat the dough for 1 minute to let some of the water evaporate. Beat in the eggs one at a time. Form éclairs on a baking sheet and bake for 22 minutes at 180°C.
Making the chocolate filling: Heat half of the cream, then incorporate 1/3 of the chocolate at a time. In a separate bowl, whip the remaining cream until firm peaks form. Fold enough of the whipped cream into the chocolate mixture until the desired consistency is obtained.
To serve, cut each éclair in half lengthwise, transfer the filling into a pastry bag and pipe it into the éclair.