Mix together 1 tbsp of flax meal and 2 1/2 tbsp of water (or any other vegan egg replacer equal to 1 egg) and set aside.
In a medium-sized bowl, mix together 1/2 cup melted vegan butter, 1 cup sugar, and 1/2 cup sifted cocoa powder until incorporated.
Add the flax egg, 2 tbsp. milk, and 2 tsp. vanilla, then blend again until fluffy. (A hand mixer works great for this or you can stir with a wooden spoon for about 2 minutes.)
Sprinkle in the dry ingredients (1 1/3 cup flour, 1 tbsp. cornstarch, 1/2 tsp. baking powder, and 1/2 tsp. salt) then mix until it forms a thick dough, and fold in the chocolate chips.
Scoop out about 2 tbsp of dough and place it on a cookie sheet. Press down to form the shape and thickness of the cookie that you want. (They won't spread too much during baking.)
Bake at 350° F (176° C) for 10-12 minutes. Take out of the oven and allow to cool on the cookie sheet for about 10 minutes before removing them with a spatula. Allow cooking on the countertop or a cooling rack.
Notes
For best results, use stick margarine. Earth Balance sticks, Mikayo, Country Crock plant-based butter or the stick version of my homemade vegan butter work best.
Melt the butter. Melted butter gives cookies a fudgy chewy texture. If you want a cakier cookie, you can use softened butter instead.
Sift the cocoa powder. Cocoa powder tends to get clumpy, especially in humid climates. To get cookies without clumps of cocoa powder, use a sifter or a metal mesh strainer to break up the clumps of cocoa before adding it to the cookies.
Be sure to spoon-measure the flour. This ensures that you are not packing the flour into the measuring cup and using too much flour in the recipe.