Preheat the oven to 350°F and adjust the oven rack to the middle position.
Start with a piece of heavy-duty aluminum foil, about 14 inches long. It should be just a little longer than it is wide. Fold the foil in half lengthwise, then fold it in half again to create a rectangle that’s about 6 inches long and 7 inches wide. Repeat to make a dozen rectangles. Place 1 sheet of folded foil with the long side facing you, place a standard-size plastic or glass spice jar on its side in the center of the foil, the jar’s long side also facing you. Bring the long sides of the foil up around the jar. The foil won’t reach all the way around, and that’s okay. Fold the foil in around both top and bottom ends of the spice jar, nice and tight. You’ll end up with 12 troughs. Spray the molds with lots and lots of nonstick spray. Place the Twinkie molds on a baking sheet or in a roasting pan and clip together to keep upright.
Whisk the flour, baking powder, and salt together in a bowl.
Heat the milk and butter in a small saucepan over low heat until the butter melts, add vanilla. Keep warm.
Separate the eggs, placing the whites in the bowl of a stand mixer fitted with the whisk attachment and beat until foamy. Gradually add 6 tablespoons of the sugar and the cream of tartar and continue to beat until the whites reach soft, moist peaks.
Transfer the beaten egg whites to a large bowl and add the egg yolks to the stand mixer bowl and beat the egg yolks with the remaining 6 tablespoons sugar on medium-high speed until the mixture is very thick and a pale lemon color, about 3 to 5 minutes. Add the beaten egg whites to the yolks, but do not mix.
Sprinkle the flour mixture over the egg whites and then mix everything on low speed for just 10 seconds. Add the melted butter mixture into the bowl and fold gently with a large rubber spatula until the batter shows no trace of flour and the whites and yolks are evenly mixed, about 8 strokes.
Immediately scrape the batter into the prepared molds, filling each with about 3/4 inch of batter. Bake until the cake tops are light brown and feel firm and spring back when touched, 13 to 15 minutes. Transfer the pan containing the molds to a wire rack and allow the cakes to cool in the molds.
Just before filling, remove each cake from the foil. Using the end of a chopstick, poke three holes in the bottom of each cake and one in each end. Wiggle the tip of the chopstick around some to make room for the filling.
Transfer the frosting to a pastry bag fit with a small tip (about 1/4 inch across). Pipe the frosting into the holes you created in the cakes. As you fill each cake, hold it in your hand and press your palm gently around it so you can feel the cake expand, taking care not to overfill and crack the cake.
SEVEN-MINUTE FROSTING RECIPE
3/4 cup sugar
1 TBSP light corn syrup
3 TBSP water
3 egg whites—lg
1/8 tsp salt
1/2 tsp vanilla
In the metal bowl of a stand mixer set over (but not touching) a saucepan of simmering water, combine the sugar, corn syrup, water, egg whites, and salt. Cook over medium heat, stirring frequently, until the sugar has completely dissolved, 3 to 4 minutes.
Attach the bowl to the mixer fitted with the whisk attachment. Beat the mixture on high speed until glossy, thick, and billowing peaks form, about 7 minutes. (It really only took about 3 to 4 minutes.) Beat in the vanilla. Use immediately.
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