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1
Set a rack in the middle of the oven and preheat to 325 degrees F. Spray a 13 by 9-inch metal pan with nonstick spray, line with parchment paper so it hangs over the sides of the pan and spray the parchment with nonstick spray.
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2
Set aside.
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3
Whisk the boiling water and cocoa powder together in a small bowl and set aside.
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4
Combine the sugar, flours, baking soda and salt in the bowl of a stand mixer fitted with the paddle attachment.
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5
Whisk the oil, sour cream, eggs and egg yolks in a large pourable vessel.
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6
Add the oil mixture to the cocoa and water mixture and slowly whisk to combine.
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7
With the mixer on low speed, add the liquid mixture to the dry mixture over 30 seconds.
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8
Continue to beat on low speed for another 30 seconds.
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9
Stop and scrape down the sides of the bowl.
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10
Continue to beat on low speed until the batter is smooth, 10 to 15 seconds.
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11
Pour the batter into the prepared pan and bake until the cake springs back when pressed and reaches an internal temperature of 205 degrees F, 30 to 35 minutes.
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12
Cool in the pan on a rack for 30 minutes, and then remove cake from the pan and cool completely before frosting, about 1 hour.
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13
Beat the butter and mayonnaise in the bowl of a stand mixer fitted with the paddle attachment, on high, until light and fluffy, 3 to 4 minutes.
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14
With the mixer on low, slowly add the melted chocolate.
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15
Continue mixing on low speed until all of the chocolate is incorporated, stopping to scrape down the sides of the bowl as needed.
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16
Turn off the mixer and add one-third of the sugar.
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17
Mix on low to combine, stopping to scrape down the sides of the bowl.
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18
Repeat until all of the sugar has been incorporated.
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19
Add the salt and continue to beat until the frosting is smooth and lightens slightly in color, 2 to 3 minutes.
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20
Use immediately or store in an airtight container at room temperature for up to 4 hours or refrigerate for up to a week.
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21
Bring to room temperature before using chilled frosting.
