Preheat the oven to 350°F (180°C) and spray two 7-inch round cake pans (see notes for different sizes*) with non-stick spray and line the bottoms with circles of parchment paper.
Sift the all purpose flour into a mixing bowl and add the brown sugar, baking soda, salt, cinnamon, nutmeg, ground ginger and allspice and mix together.
Prepare a flax egg by adding 1 Tbsp ground flaxseed meal to a bowl and then adding 3 Tbsp hot water. Let it sit for a minute to become gloopy.
Add the flax egg, soy milk, unsulphured molasses, vanilla extract, oil and apple cider vinegar to the mixing bowl and whisk briefly with a hand whisk to combine. Don't overmix.
Divide the batter evenly between your two prepared 7-inch cake pans.
Bake at 350°F for 25-30 minutes or until a toothpick inserted into the center of one of the cakes comes out clean.
Let the cakes cool for a few minutes and then transfer them to a wire cooling rack to cool completely before frosting.
Prepare your frosting by adding the vegan butter, powdered sugar and 3 Tbsp orange juice to the bowl of an electric mixer. Starting at slow speed, gradually increase speed until thick and smooth. If it's too thick add in the remaining 1 Tbsp orange juice, but add it in slowly as you may or may not need all of it.
Place the frosted cake in the fridge for an hour so that the frosting can set before slicing and serving.