Preparation

• Preheat the oven to 350°F (175°C) and adjust the oven rack to the lower-middle position.
• To make your shiny, single-use Twinkie molds, start with a piece of aluminum foil, preferably heavy-duty, that’s approximately 14 inches long. 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 on your work surface, with the long side facing you. Place a standard-size plastic or glass spice jar on its side in the center of the foil. Bring the long sides of the foil up around the jar. Fold the foil in around both top and bottom ends of the spice jar. Repeat until you have 12 foil Twinkie molds. Spritz the molds with nonstick spray or coat with vegetable oil.
• Whisk the flours, baking powder, and salt together in a bowl.
• In a small saucepan over low heat, heat the milk and butter until the butter melts. Remove from the heat and add the vanilla. Cover to keep warm.
• Separate the eggs, placing the whites in a standing mixer bowl and reserving the yolks. Beat the whites on high speed until foamy. Gradually add 6 tablespoons of the sugar and cream of tartar, continue to beat until the whites reach soft peaks.
• Dump the beaten egg whites into a large bowl and add the egg yolks to the standing mixer bowl. Beat the yolks with the remaining 6 tablespoons sugar until thick and pale, about 5 minutes. Add the beaten egg whites to the yolks without mixing.
• Sprinkle the flour mixture over the egg whites and mix on low for 10 seconds. Pour the melted butter mixture into the bowl and fold gently until the batter shows no trace of flour, about 8 strokes.
• Immediately scrape the batter into the prepared molds, filling each with about ¾ inch batter. Bake until light brown and spring back when touched, about 13 to 15 minutes. Allow the cakes to cool in the molds.
• Remove each cake from the foil. Poke three holes in the bottom of each cake with a chopstick. Wiggle to create room for filling.
• Scoop frosting into a pastry bag fit with a small tip and pipe into the holes in the cakes. Fill gently without overfilling.
• Serve while still slightly warm.