Preheat oven to 350 degrees F (175 degrees C). Grease a 7x11-inch baking dish with coconut oil and line with parchment paper.
Combine maple syrup, coconut milk, mayonnaise, coconut sugar, and vanilla extract in a bowl; whisk well until combined.
Mix cassava flour, cocoa powder, baking soda, and salt in a separate bowl. Pour maple syrup mixture into flour mixture; whisk until well combined and a smooth batter is formed, about 5 minutes. Pour batter into the prepared dish.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 35 minutes. Let cool completely in the baking dish.