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Preheat oven to 350 F and grease eleven 8 ounce jelly jars with butter.
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For the custard:
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Whisk together the egg, egg yolks, and vanilla in a medium-sized heat proof bowl and set aside.
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Whisk together the milk, sugar, flour, and salt in a medium saucepan. Bring to a gentle boil over medium heat, stirring occasionally. Turn off the heat and very slowly ladle the hot milk mixture into the egg mixture while continually whisking. Add just a drop at a time at first, then whisk in the butter and slowly add the rest of the hot milk mixture. Strain the custard into a bowl through a fine mesh sieve to remove any lumps. While it's still hot, slowly pour the custard into the prepared jars, being careful not to splash the sides of the jar. Cool the custard a bit (so that it's set) before you pour the cake batter on top.
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5
For the cake:
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While the custard cools, make the cake batter. Whisk together the butter, eggs, sugar, vanilla, and milk in a large bowl. Set aside.
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In a separate bowl, whisk together the flour, baking powder, and salt.
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In another bowl, whip the egg whites to stiff peaks. Whisk the dry ingredients into the wet, then fold in the egg whites, 1/3 at a time. Carefully pour the batter onto the top of the set custard (you can use a small ladle or put the batter into a pastry bag and pipe it).
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Arrange the jars (so that they don't touch each other or the sides of the pan) in a large, deep roasting pan. Add enough hot (not boiling) water into the roasting pan to come up to the top of the custard part of each jar (but not the cake batter). Bake until the cake is light golden on top and a toothpick inserted into the cake inside comes out clean or with just a couple crumbs, about 45 minutes. Cool completely before proceeding.
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For the dark chocolate ganache: Melt the chocolate and butter together in a microwave or in the top of a double boiler. If you heat it in the microwave, make sure to heat for only 30 seconds at a time and stir between each additional 30 seconds. Add the milk and vanilla into the melted chocolate mixture and stir until smooth. Spoon over the cooled cakes, and let the chocolate set before piping the buttercream on top.
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For the white chocolate buttercream: Melt the white chocolate in a microwave or in the top of a double boiler. If you heat it in the microwave, make sure to heat for only 30 seconds at a time and stir between each additional 30 seconds. Beat together the melted chocolate, butter, shortening, and vanilla. Slowly beat in the powdered sugar. While still beating, add the milk (a little at a time) until the frosting reaches your desired consistency. Pipe the frosting on top of the cakes once the ganache is set.
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Serving and storing: Serve these within 1 to 2 hours of having them fully assembled. Otherwise, you can make them up to 3 days ahead of time. If you do, store them in the fridge and let them sit at room temperature for about an hour before serving.
