Preheat the oven to 180°C. Lightly grease a 23x23cm/9x9in square pan with a little bit of oil or vegan butter. Then line the pan with a strip of parchment paper.
To make vegan buttermilk, add a tablespoon of apple cider vinegar (or lemon juice) to unsweetened almond milk. Mix well. Set aside.
In a mixing bowl, beat together with a hand mixer or whisk brown sugar, sunflower oil, molasses, and apple sauce for about 1 minute. Add vegan buttermilk and mix again until combined.
In a separate bowl, combine all-purpose flour, cocoa powder, gingerbread spice mix, baking soda, baking powder, and sea salt.
Sift the flour mixture into the large bowl with the wet ingredients and use a whisk to mix the batter until just combined. Don’t overmix the batter.
Pour the batter into the prepared pan and spread it to the sides.
Bake in preheated oven for 20-25 minutes until a toothpick inserted into the middle comes out clean. Otherwise, bake for 2-5 minutes longer.
Let the cake cool completely before frosting.