To make the cake: Position a rack in the lower third of the oven and preheat the oven to 350 F. Grease the sides of the cake pans with vegetable oil spray or butter and line the bottoms with 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. Add the eggs, milk, and vanilla and beat on medium speed for 2 minutes. Add the oil and beat until smooth. Stir in the hot water until well incorporated. The batter will be thin.
Divide the batter between the prepared pans and bake 8 inch layers for 20-25 minutes or 9-inch layers for 30-35 minutes until the cakes pulls way 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.
When the layers are cool, slide a thin knife or a small metal spatula around the edge of the cakes to detach them from the pans. Invert the layers and peel off the parchment liners. Set on layer on a serving platter. For a two-layer cake, spread one quarter of the frosting on top of the 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 half an hour. 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. The cake keeps in an airtight container for up to 5 days in the refrigerator. Bring to room temperature before serving.
To make the frosting: Put the cream and chocolate in a medium stainless steel owl. Bring and inch of water to s 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 in chunks and whisk once or twice to break them up; let the mixture rest for 5 minute to finish melting the butter before whisking it smooth. Taste and add salt if desired. Set aside, without mixing or disturbing, to cool and thicken (this takes at least a couple of hours). Leftovers keep, covered, in the refrigerator for up to 5 days.