Preheat the oven to 350°F. Prepare cake pan as follows: coat with softened coconut oil, then dust completely with cocoa powder. Set aside.
In a large bowl, sift together the dry ingredients: oat and almond flour, cocoa powder, baking soda and salt. Set aside.
Add water and milk to a microwave-safe bowl. Heat in 10-second increments until just warm. This will prevent the melted coconut oil from solidifying once mixed with these liquids.
In a medium bowl, whisk together all wet ingredients: water, milk, coconut oil, sugar, maple syrup and vanilla. Whisk until well incorporated. Add wet ingredients to dry ingredients. Whisk until just incorporated, making sure no flour patches remain.
Pour batter evenly into prepared cake pan. Bake for 22-26 minutes. Test for doneness with a toothpick—it should come out clean when done. Allow cake to cool completely before frosting (about 1 hour).