Step 1: Set the oven to 400°F (or 350°F for convection) and lightly butter two large baking sheets.

Step 2: In a saucepan, combine the butter and water; stir occasionally and bring to a boil. Take off the heat and quickly add the flour, beating until the dough comes together into a ball.

Step 3: After allowing it to cool slightly, gradually beat in the eggs to create a glossy, smooth paste.

Step 4: Spoon the mixture into a piping bag that has a ½-inch plain pipe fitted in it. Arrange 20 éclairs, approximately 2 inches in length, onto the baking sheets, allowing space for them to grow as they bake. Bake for about 20 minutes, or until golden brown and well-risen, in a preheated oven.

Step 5: Take out of the oven and use a knife to crack up one side of each éclair so that the steam may escape.

Step 6: Place on a wire rack to cool. Return the split éclairs to the oven for an additional five minutes to dry them out if the insides are still damp.

Step 7: Spoon or pipe the whipped cream into the center of the éclair bases, then replace the tops.

Step 8: Sift the cocoa and confectioners’ sugar into a bowl, then add the rum and just enough warm water to produce a thick frosting.

Step 9: Using a fork, spear each éclair and dip the tops into the frosting. On the day they are made and filled, let them set and serve.