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Preheat the oven to 350°F. Spritz a filled cake pan (aka canoe panel pan) with a light layer of cooking spray. Set aside.
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To make the snack cakes: Place all ingredients except beaten egg whites into a large mixing bowl. Beat until smooth.
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Working in thirds, fold the beaten egg whites into the batter. Take care to keep the batter light; fold, don't beat.
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Fill each section of the snack cake pan 2/3 full. The cakes will puff quite a bit during baking, but will shrink back during cooling. Bake the cakes in a preheated oven for 8-12 minutes, until golden brown.
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Remove the pan from the oven and let cool for 5 minutes. Using a flexible spatula, gentle remove each cake and cool flat side down on a rack until completely cool. In the meantime, prepare the snack cake filling.
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In a small saucepan cook flour and milk over medium heat until a paste forms. Stir constantly and do not allow mixture to brown. Remove from heat and let cool 1 minute. Add vanilla and stir until smooth. Press a piece of plastic wrap down on the surface of the paste to avoid forming a skin and set aside to cool completely.
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In the bowl of your mixer beat butter, shortening, and sugar until fluffy, scraping bowl to fully incorporate ingredients. Add cooled flour/milk mixture and continue to beat 5 minutes on medium-high speed until smooth and creamy. Use to fill snack cakes or cupcakes. Filling will remain creamy; store at room temperature 3-4 days.
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To fill snack cakes: Place about 1 cup of filling into a clean pastry bag fitted with a round pastry tube, or use filling tool included with snack cake (cream canoe) pan. Gently insert the tube into the underside of the cake, about halfway through the cake. Using gentle pressure, squeeze a small amount of filling into the cake. You'll feel the cake expand under your fingers. Do not overfill, or the cake will burst. Repeat for a total of 3 times per cake. Store cakes well covered at room temperature for up to 4 days.
