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1
Preheat the oven to 350 degrees F. Thoroughly spray a large (18-by-12-inch) sheet cake pan (or you can use a slightly smaller jelly roll pan) with nonstick baking spray.
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2
Be sure to get in all the corners of the pan.
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3
Sift together the cake flour and salt.
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4
Set aside.
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5
In a separate bowl, stir together the buttermilk, vanilla, baking soda and eggs.
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6
Add the vinegar and stir.
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7
Set aside.
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8
In a separate small bowl, mix together the cocoa and red food coloring.
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9
Set aside.
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10
Cream together the shortening and sugar until fluffy.
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11
Alternate adding the flour mixture and the buttermilk/egg mixture until all combined.
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12
Pour in the red mixture and beat until combined.
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13
Pour the batter into the prepared sheet cake pan.
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14
Use a large offset spatula or knife to even out the surface.
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15
Bake for 20 minutes.
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16
Remove the pan from the oven and allow the cake to cool in the pan for 20 minutes.
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17
Place a large cutting board on top of the cake pan, then invert the cake, allowing it to turn out onto the cutting board.
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18
Allow the cake to cool completely before icing.
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19
Combine the milk and flour in a saucepan over medium heat and whisk constantly until thick.
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20
Remove from the heat and let cool to room temperature.
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21
Add the vanilla once cooled.
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22
In the bowl of a stand mixer with the paddle attachment, cream the butter and sugar until light and fluffy.
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23
Add the cooled milk mixture and beat very well.
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24
It may look separated, which means you have to keep beating until fully combined.
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25
It should be fluffy and light, similar to whipped cream.
