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1
Heat the oven to 350F.
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2
Combine the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes.
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3
Scrape down the sides of the bowl, add the eggs, and mix on medium-high for 2 to 3 minutes.
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4
Scrape down the sides of the bowl once more.
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5
On low speed, stream in the buttermilk, oil, and vanilla.
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6
Increase the mixer speed to medium-high and paddle for 3 to 5 minutes, until the mixture is practically white, twice the size of your original fluffy butter-and-sugar mixture, and completely homogenous.
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7
There should be no streaks of fat or liquid.
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8
Stop the mixer and scrape down the sides of the bowl.
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9
Add the fudge sauce and mix on low speed until fully incorporated.
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10
Scrape down the sides of the bowl.
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11
With a spatula, stir the flour, cocoa powder, baking powder, and salt together in a medium bowl.
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12
On very low speed, add the dry ingredients and mix for 45 to 60 seconds, just until your batter comes together.
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13
Scrape down the sides of the bowl, and mix on low speed for another 45 seconds to ensure that any little lumps of cocoa powder and cake flour are incorporated.
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14
Pam-spray a quarter sheet pan and line it with parchment, or just line the pan with a Silpat.
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15
Using a spatula, spread the cake batter in an even layer in the pan.
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16
Bake for 30 to 35 minutes.
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17
The cake will rise and puff, doubling in size, but will remain slightly buttery and dense.
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18
At 30 minutes, gently poke the edge of the cake with your finger: the cake should bounce back slightly and the center should no longer be jiggly.
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19
Leave the cake in the oven for an extra 3 to 5 minutes if it doesnt pass these tests.
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20
Take the cake out of the oven and cool on a wire rack or, in a pinch, in the fridge or freezer (dont worry, its not cheating).
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21
The cooled cake can be stored, wrapped in plastic wrap, for up to 5 days.
