1. Preheat oven to 350 degree F. Lightly grease a Twinkie cake pan and set aside. 
2. In a large bowl, whisk together the all-purpose flour, sugar, baking powder and salt until combined. Set aside.
   105 grams all-purpose flour, 150 grams sugar, divided, 4 grams baking powder, 2 grams salt
3. Separate the room temperature egg yolks and egg whites in two bowls.
   5 eggs
4. In a large glass measuring cup, mix the sourdough discard, oil, 3 egg yolks, and vanilla extract. Pour the wet ingredients into the dry ingredients and mix until smooth and combined. 
   120 grams sourdough discard, 4 grams vanilla, 60 grams neutral oil
5. In a bowl of a stand mixer with the whisk attachment, beat the 5 egg whites and cream of tarter on high speed until soft peaks form (about 1-2 minutes).

   Cream of tarter helps the egg whites remain stiff, which helps the cakes rise while baking. However, it can be omitted, if preferred. See Notes below.
   3 grams cream of tarter
6. Lower the speed to low and slowly pour 50 grams (1/4 cup) sugar. Turn the speed to high and continue to beat the egg whites until stiff peaks form (1-2 additional minutes).
7. With a rubber spatula, mix a third of the egg whites in the cake batter to create a slightly looser consistency, Then, gently fold the remaining egg whites in the batter until the whipped whites just disappear.
8. Scoop the batter into a large pastry bag with a large pastry tip. Pipe the Sourdough Twinkie batter into the prepared baking pan 2/3 the way full. 
9. Bake for 15-20 minutes, or until a toothpick comes out clean and the Twinkies are golden brown. Place a piece of parchment paper on top of the baking pan, flip it over, and gently tap the pan to release the baked Sourdough Twinkies. Leave to cool completely before filling each Twinkie with the creamy filling. 

Vanilla Buttercream

1. In a bowl of a stand mixer with the whisk attachment, whip the room temperature butter on medium-high speed for 2-3 minutes, or until light and creamy.
   42 grams unsalted butter, room temperature
2. Lower the speed to medium-low and gradually add the powdered sugar at a time. Once fully combined, add vanilla extract, heavy cream, and a pinch of salt and mix. Stop to scrape the sides of the bowl to ensure all the ingredients are mixed fully. 
   100 grams powdered sugar, 2 grams vanilla, 15-30 grams heavy cream, pinch of salt

Assemble

1. Fit a large pastry bag with a medium piping tip and fill it with the frosting. 
2. Inject the frosting from the bottom of each Sourdough Twinkie Cake by inserting the tip into it about 3/4 of the way through. Fill each hole with the Twinkie filling until the cake begins to bulge and the frosting pours out. Repeat injecting the vanilla buttercream every 1/4 -1/2 inch.