Special equipment: A large pastry bag fitted with a large star tip, an electric mixer fitted with a paddle attachment
Preheat the oven to 425 degrees F.
Sift the cocoa powder and flour together at least 3 times. Set aside. Place the water, butter, sugar, and salt in a heavy 2-quart saucepan. Bring this mixture just to a boil over high heat. Remove from heat and add the sifted cocoa and flour in all at once. Stir with a wooden spoon until the dough comes together to form a ball. Return the pan to medium-high heat and beat constantly until a film develops on the bottom of the pan, about 2 minutes. Do not let it burn.
Transfer the dough to the bowl of an electric mixer with a paddle attachment. Add the eggs 1 at a time, beating well after each addition. Scrape down the side of the bowl and continue to beat the dough for 4 minutes at high speed. Transfer the dough into a piping bag fitted with a large star tip. Line a baking sheet with parchment paper and pipe 3 to 4-inch log shapes onto the paper. Bake for 10 minutes; then reduce the oven temperature to 325 degrees F. Bake for another 15 minutes. Turn the oven off. Remove the eclairs from the oven and poke a hole in each one using a skewer or piping nail. Place the eclairs back in the oven and leave the door ajar for 10 minutes, allowing the eclairs to dry.
Cut the eclairs in 1/2 lengthwise with a serrated knife. Place the bottom halves of the eclairs on a sheet pan and put 3 marshmallows into each. Place under the broiler until golden, watching closely. Remove from oven. Place tops on and ladle the chocolate sauce over top.
Place the chocolate in a stainless-steel bowl. Pour the heavy cream into a small saucepan over medium-high heat and heat the cream until just before the boiling point. Pour the cream over the chocolate and let sit for 1 minute. Whisk until smooth and glossy.