Prepare the flax egg by mixing 1 tablespoon of flaxseed meal with 2.5 tablespoons of water in a small bowl. Let it sit for about 5 minutes until it thickens.

In a large mixing bowl, beat together 1/2 cup of vegan butter, 1/2 cup of brown sugar, and 1/4 cup of granulated sugar using an electric mixer on medium speed for about 2-3 minutes.

Add the prepared flax egg, 2 tablespoons of espresso, and 1 teaspoon of vanilla extract to the creamed mixture. Mix everything together on low speed until fully combined.

In another bowl, whisk together 1 cup of all-purpose flour, 1 teaspoon of baking powder, 1/2 teaspoon of baking soda, 1/4 teaspoon of salt, and 1/4 cup of cocoa powder. Gradually add this mixture to the wet ingredients.

Cover the dough with plastic wrap and refrigerate for about 20 minutes.

Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper. Scoop portions of the cookie dough and space them about 2 inches apart.

Bake for 10-12 minutes until the edges are set and the centers are soft.

Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack. If using the optional filling, mix together 1/2 cup of vegan cream cheese with 1/4 cup of powdered sugar, and spread it between two cookies.