Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius) and grease an 8-inch round cake pan.
Sift together whole wheat pastry flour, cocoa powder, baking soda, and sea salt in a large mixing bowl.
In a separate bowl, whisk together unsweetened applesauce, maple syrup, eggs, and vanilla extract until fully emulsified.
Incorporate the wet ingredients into the dry ingredient mixture, stirring until just combined.
Carefully pour the boiling water into the batter and whisk gently by hand until the mixture is smooth and aerated.
Pour the batter into the prepared cake pan and bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.