Preheat oven to 350°F or 175°C.
In a small bowl, add the coffee, oatmeal, and boiling water.
Cover with a paper towl and let it sit for 20 minutes.
Meanwhile, prepare your flax eggs by mixing the flax seeds and water in a cup and letting it sit for 15 minutes.
In a large bowl, cream both sugars and oil.
Pour in the flax eggs and vanilla and mix until combined.
In a medium bowl, whisk the flour, baking soda, salt, and cocoa.
Pour the oatmeal mixture into the large bowl of wet ingredients and mix.
Pour the dry ingredients into the bowl of wet ingredients and mix until combined.
Grease a 9x13 pan and pour in the batter.
Bake for 50 minutes or until a toothpick comes out clean when inserted in the cake.