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1
Make the custard filling:
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2
In a bowl, beat egg yolks and whole egg. Set aside.
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3
In another bowl, whisk milk, sugar, and flour until combined and no lumps remain.
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4
In a double boiler (or a sauce pan over low heat), melt butter. Once butter has melted, pour in milk mixture and bring to a slow boil. Stir constantly. Remove from heat once mixture has thickened. Add in eggs and vanilla, and whisk until combined. Bring back to the stove top, and let custard thicken over low heat, around 5-10 minutes. Remove from heat and cool the mixture before using.
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5
Make the cupcakes while the custard cools.
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6
Preheat oven to 175u00b0C. Line a muffin tin with cupcake liners.
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7
In a large bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
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8
In a mixing bowl, beat melted butter and sugar until combined. Add in egg whites, and beat until smooth and pale. Add in vanilla. Add in half of the flour mixture, then add the sour cream, and then add the remaining flour mixture. Scrape down the sides of the bowl as needed. Beat until everything is combined. Do not overbeat!
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9
Scoop batter into prepared tins, just a spoonful or two of the cupcake batter. Then, using a spoon, pour in a dollop of custard filling, then scoop batter once again to cover the filling. Don't worry about the custard sinking to the bottom, it literally floats in the middle.
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10
Bake at 175u00b0C for 20 to 25 minutes or until tops spring back when tapped.
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11
Remove from the oven, and cool on a wire rack. Once the cupcakes are cooled, you can put them in the fridge to let the custard settle.
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12
Frost with your favorite frosting. I left mine plain and simple.
