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1
Position a rack in the lower third of the oven and preheat the oven to 350u00b0 F. Grease a 9- by 13-inch metal or glass baking pan.
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2
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.
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3
Scrape the batter into the pan and bake for 30 to 35 minutes in the metal pan or a little longer in glass, until the cake pulls away slightly from the edges of the pan and a toothpick inserted near the center comes out almost clean. Set the pan on a rack to cool completely before frosting or storing.
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4
Make the frosting: Put the cream, chocolate, and salt in a medium stainless steel bowl. Bring an inch of water to a simmer in a wide skillet.
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5
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.
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6
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.
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7
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 adjust salt if necessary. Set aside, without mixing or disturbing, until needed.
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8
If the frosting is stiff by the time you need it, set the bowl in a pan of hot water to soften it, stirring occasionally until smooth and just pourable. Scrape the frosting over the cake and spread it evenly. Let set at room temperature, then cover and refrigerate the cake or keep it at room temperature-it's great either way. The cake keeps for at least 3 days at room temperature and 5 days in the refrigerator.
