Combine ground flax seed with hot water and whisk smooth
Let stand for at least 5 minutes to thicken to a paste
In the meantime, cream the softened vegan butter with the light brown sugar on medium -high speed until very light and fluffy, about 3-5 minutes.
Scrape bowl periodically to ensure even mixing
Sift together all the dry ingredients from the baking soda to the cocoa powder and reserve.
Add the molasses to the hot water
Add the flax paste to the creaming vegan butter and sugar mixture, and mix well.
Add ⅓ of the dry ingredients, blend well
Then add ½ of the molasses/water mixture slowly while mixing on low speed
Stop and scrape the bottom and sides of the bowl often
Repeat the process with another ⅓ of the dry, the remaining liquids and then the last addition of dry ingredients
Pour the batter into a greased and parchment lined 8" cake pan and bake immediately in preheated 350° F oven approximately 30-40 minutes or when it is springy to the touch when you gently press the center or a toothpick inserted comes out clean
Recipe by Gretchen's Vegan Bakery at https://www.gretchensveganbakery.com/pumpkin-mousse-cake/