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1
Heat the oven to 350F.
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2
Combine the butter, shortening, and sugars 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 4 to 6 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
Dont rush the process.
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8
Youre basically forcing too much liquid into an already fatty mixture that doesnt want to make room for the liquid.
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9
There should be no streaks of fat or liquid.
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10
Stop the mixer and scrape down the sides of the bowl.
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11
On very low speed, add the cake flour, baking powder, salt, and the 50g (1/4 cup) rainbow sprinkles.
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12
Mix for 45 to 60 seconds, just until your batter comes together.
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13
Scrape down the sides of the bowl.
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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
Sprinkle the remaining 25g (2 tablespoons) rainbow sprinkles evenly on top of the batter.
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17
Bake the cake for 30 to 35 minutes.
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18
The cake will rise and puff, doubling in size, but will remain slightly buttery and dense.
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19
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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20
Leave the cake in the oven for an extra 3 to 5 minutes if it doesnt pass these tests.
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21
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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22
The cooled cake can be stored in the fridge, wrapped in plastic wrap, for up to 5 days.