Preheat oven to 350 degrees and line a 9 x 13 pan with parchment paper. Leave enough paper up the edges to lift the cake out of the pan. Set aside.
Whisk together sifted flour, salt, baking soda, and baking powder in a large bowl. Set aside.
Add spinach and non dairy milk to blender. Blend until spinach is completely broken down. If you don't have a high speed blender you may want to strain the liquid to remove any spinach bits that remain.
Add vinegar, maple syrup, applesauce, coconut oil, and vanilla. Blend again to combine.
Make a well in the center of flour mixture and pour in wet ingredients. Stir together to combine. Take careful not to over mix. It's ok if small lumps remain.
Pour batter into prepared cake pan and bake for 20 minutes or until when pressed lightly the cake springs back and a toothpick inserted comes out clean.
Allow to cool for a 10 minutes before lifting the parchment paper to remove the cake. Move to a cooling rack to cool completely before cutting.
Once the cake has cooled cut edges off to remove the cake that browned a bit while baking. Then cut cake into 8 equal rectangles. You can also cut into 16 rectangles to make mini cakes, if you prefer.
Decorate as desired to create Frankenstein's face.