Position a rack in the lower third of the oven and preheat to 350 degrees Fahrenheit. Grease the sides of the cake pans with nonstick cooking spray or butter and line the bottoms with Gefen Parchment Paper.

Put the sugar, rice flour, oat flour, cocoa powder, salt, xanthan gum, baking powder, and baking soda in the bowl of the stand mixer and mix with the paddle attachment until well combined.

Divide the batter between the prepared pans and bake eight-inch layers for 20 to 25 minutes or nine-inch layers for 30 to 35 minutes, until the cakes pull away slightly from the edges of the pans and a toothpick inserted near the center comes out almost clean. Set the pans on a rack to cool completely before frosting or storing.

Add the eggs, milk, and vanilla and beat on medium speed for two minutes. Add the oil and beat until smooth. Stir in the hot water until well incorporated. The batter will be thin.

Put the cream and chocolate in a medium stainless steel bowl. Bring an inch of water to a simmer in a wide skillet. Turn off the heat and set the bowl of chocolate in the water. Let it rest for 15 minutes, gently shaking the bowl several times to submerge the chocolate in the cream.

When the chocolate is melted, start whisking at one edge and continue whisking until all of the chocolate is incorporated and the mixture is smooth. Add the butter or coconut oil in chunks and whisk once or twice to break them up; let the mixture rest for five minutes before whisking it smooth. Yield: 1 quart

When the cake layers have cooled, slide a thin knife or a small metal spatula around the edges to detach them from the pans. Invert the layers and peel off the parchment.

Set one layer on a serving plate. For a two-layer cake, spread one quarter of the frosting on the first layer and top with the second layer. For a three-layer cake, spread one-fifth of the frosting on the first layer, top with the second layer, spread with frosting, and top with the third layer. Spread a very thin layer of frosting over the top and sides of the cake, just to smooth the surface and glue on any crumbs. Chill to set the frosting, about 30 minutes.

Stir the remaining frosting until smooth (warm it slightly if necessary) and spread it over the top and sides of the cake, as luxuriously as you want.