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1
Grease a 10 1/2-by-14-by-2-inch pan and set aside.
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2
Prepare cake mix as instructed on the box using the egg and oil.
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3
Spread into reserved pan and bake in a preheated 350-degree oven for 20 minutes.
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4
Remove from oven and place pan on a wire rack and let cake cool completely in the pan.
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5
Prepare filling: Place flour in a small saucepan.
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6
Slowly add milk, stirring until mixture is smooth.
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7
Place pan over medium heat and cook, stirring constantly, until mixture is thick, about 10 minutes.
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8
Place flour-mixture in a bowl, cover surface with plastic wrap and refrigerate until cool, about 2 hours.
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9
In a bowl of an electric mixer, combine the cooled flour mixture, margarine, vegetable shortening and sugar.
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10
Beat on high speed until light and fluffy, 7 to 8 minutes.
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11
Spread evenly over top of cake and chill until filling is somewhat firm, about 2 hours.
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12
Prepare icing: In a large bowl of an electric mixer, combine melted margarine, egg, melted chocolate, 2 1/2 tablespoons hot water and vanilla.
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13
Beat on medium speed until blended.
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14
Slowly add confectioners' sugar, beating until smooth.
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15
Smooth icing over cake, taking care not to disturb the filling.
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16
Chill until ready to serve.
