To a mixing bowl, add the all-purpose flour, cocoa powder, baking soda and salt. Whisk the dry ingredients and set aside.
Cream together the granulated sugar and the vegan butter inside the bowl of a stand mixer, with the paddle attachment on a medium speed. When the mixture begins to combine, pour in the soy milk and vanilla extract. Keep creaming until fully combined and smooth.
Gradually, add the dry mixture to the stand mixer bowl and turn down to a low speed while the mixture comes together. Once this happens, switch back to a medium speed until a dough ball can be formed with your hands.
On a large, well-floured surface, roll out your cookie dough to about one-eighth of an inch thick (3.18mm). Use a 7.5cm x 4cm rectangular cookie cutter to cut out the cookies This should yield about 35-40 small chocolate wafer cookies.
Carefully place the cut-out cookie dough in the fridge to chill for 25 minutes.
Remove the unbaked cookies from your fridge. Place them onto the lined baking trays about 1-inch apart and bake in the preheated oven for 10-12 minutes. Let cookies cool fully on a cooling rack.
In the meantime, line an 8x8 inch square baking tin with parchment paper that overhangs so that the cake can easily be removed.