• Preheat the oven to 350°F. Line two 9x13 cake pans with parchment paper, leaving a slight overhang for easy removal. Then lightly spray the parchment paper with nonstick cooking spray. Set aside. 

  ^{*You can also skip the parchment paper and spray each pan really well with nonstick spray.}

Yellow Sponge Cake

• In a large mixing bowl, add the dry yellow cake mix, whole milk, vegetable oil, large eggs, large egg yolk, and vanilla extract. Beat with a hand or stand mixer on medium speed for 2 to 3 minutes, until smooth and slightly thick. 
  1 box (15.25 oz) yellow cake mix, 1 cup whole milk, 1/2 cup vegetable oil, 3 large eggs, 1 large egg yolk, 1 teaspoon pure vanilla extract
• Divide the batter evenly between the two prepared cake pans, spreading each portion into an even layer. It will be thin and that's ok. 
• Bake both cakes on the center rack in the oven for 15-20 minutes, until the tops are light golden and a toothpick inserted in the center comes out clean with just a few moist crumbs. 
• Remove the cakes from the oven and let them cool completely in the pans. About 1 hour. 

Marshmallow Cream Filling

• In a large bowl, beat the softened butter on medium speed for 1-2 minutes until smooth, pale, and creamy. 
  1¼ cups unsalted butter
• Add the marshmallow fluff and continue beating on medium speed until the ingredients are fully combined and fluffy, about 1 minute. 
  1 jar (7 oz) marshmallow fluff
• Add the sifted powdered sugar and beat on low speed, increasing the speed as needed, until it's smooth. Scrape down the sides of the bowl as needed. 
  4 cups powdered sugar
• Add 3 tablespoons heavy cream, vanilla extract, and salt. Continue beating on medium speed until the filling is light, fluffy, and holds its shape.

  ^{*Add the remaining 1 tablespoon heavy cream only if needed to thin the frosting more for spreadability.}
  3-4 tablespoons heavy whipping cream, 1¼ teaspoons pure vanilla extract, ¼ teaspoon sea salt

Assemble The Twinkie Cake

• Lift one completely cooled cake layer out of the pan, using the parchment paper overhang or invert the pan and the cake should fall right out, and place it on a serving platter, cake platter, or just leave the cake inside the cake pan for serving/storage/refrigerating (or whatever you want to keep it on for serving).

  ^{*I leave the one cake inside the pan, add the cream filling, and then remove the other cake from the pan and place it on top.}
• Then spread the marshmallow filling evenly over the surface in a thick, generous layer, leaving a small border around the edges. Carefully place the second cake layer on top of the filling, and very gently press it down so it stays in place.
• You can serve the cake immediately OR cover the cake with plastic wrap and refrigerate for up to 6-8 hours (or overnight) before serving.

  ^{*I prefer refrigerating the cake before serving so I spread the filling over the cake layer inside the cake pan, and then place the other cake layer on top of it. I then cover the cake pan with a lid or plastic wrap and refrigerate.} 

  ^{*Because I usually have leftover heavy whipping cream (the recipe only calls for a few tablespoons) I like to use the remaining heavy whipping cream to make homemade whipped cream for serving on top of each slice. Add 1.5 cups heavy whipping cream + 1/3 cup powdered sugar to a bowl and beat until stiff peaks form.}