Preheat the oven to 350 degrees Fahrenheit. Line a large sheet pan with a piece of parchment paper.
Add the flax meal and water into a small bowl. Stir well and let it sit for 10 minutes. This is called a flax egg.
Place a large saute pan over medium heat. Once hot, add in the chopped walnuts and cook them for two minutes, tossing them around frequently to toast them evenly. Pour them in a bowl to cool down, and set it aside.
Add the butter into a small saucepot and place it over low heat. Once the butter is entirely melted, take it off the heat and stir in the cocoa powder until evenly combined and there are no lumps.
Add the sugars and melted butter mixture into the bowl of a standing mixer with the paddle attachment. Alternatively, you can add the ingredients into a large mixing bowl and use a handheld beater instead. Beat the mixture over medium speed for two minutes.
Add the flax egg into the mixer and beat over medium speed for another two minutes.
Combine the all-purpose flour, baking soda, and salt in another mixing bowl. Whisk well. Then, slowly dump it into the standing mixer while it is beating over the lowest speed. Beat the ingredients until just combined. Do not over-mix.
Now, add the chocolate chips, marshmallows, and toasted walnuts into the mixer. Stir them into the dough using a rubber spatula or a wooden spoon until evenly combined.
Roll two tablespoon portions of the dough between your hands. Ensure you press the chocolate chips, marshmallows, and walnuts securely into the ball of dough; otherwise, they will fall off, and the cookie also tends to break apart more.
Place six cookie dough balls on the prepared baking sheet at least two inches apart from each other. Rest the sheet pan on the middle oven rack and bake the cookies for 9-10 minutes.
Place a piece of plastic wrap onto the cookie dough that is still in the mixing bowl to prevent it from drying out while the cookies in the oven are baking.
Allow the cookies to mostly cool on the sheet pan before moving them. They are somewhat fragile when hot.
Repeat the last four steps of the recipe until you have no cookie dough left!