Preheat oven to 350 degrees Fahrenheit. Grease two 9-inch cake pans with coconut oil and line bottoms with parchment paper.
In a large bowl, whisk together sweet rice flour, millet flour, tapioca starch, sorghum flour, cocoa powder, coconut sugar, baking soda, baking powder, and salt. In a medium bowl, whisk together melted coconut oil, buttermilk, eggs, and vanilla extract.
Add liquid mixture to the dry ingredients mixing with an electric mixer on low. Add hot water and beat until just combined. Divide batter between both pans and bake for 28 to 30 minutes or until a toothpick comes out clean. Cool in cake pans on wire rack.
To make frosting:
In a medium bowl, whip palm shortening for about 30 seconds. Then add pumpkin, maple syrup, vanilla extract, and spices. Mix until fully combined.
To assemble cake:
Once cakes are completely cool, on a cake plate or stand, place one cake top side down. Spread a layer of frosting on top. Place second cake top side down on top of bottom layer. Frost a thin crumb coat on top and sides of cake. Refrigerate for about 15 minutes and then frost the remainder of sides and top of cake. Sprinkle cacao nibs around top perimeter of cake.