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Using a blender, mix 2 cups flour, 2 cups milk, 4 eggs, 1/2 teaspoon vanilla and 1/2 teaspoon salt until smooth; refrigerate.
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In a medium saucepan, whisk together the 1 cup heavy cream, remaining 1-1/4 cups milk, 1/2 cup sugar and 1 pinch salt.
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Cook over medium heat until steaming, 2 minutes.
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In a medium bowl, whisk together the remaining 2 eggs and 1/2 cup sugar.
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Whisk in the remaining 2/3 cup flour until smooth.
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Gradually whisk half of the hot cream mixture into the flour-egg mixture, then whisk into the remaining cream mixture.
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Cook, whisking, over medium heat until thick, about 2 minutes; stir in the rum and the remaining 1 teaspoon vanilla.
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Scrape the pastry cream into a bowl and refrigerate, stirring occasionally, for 20 minutes.
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In an 8-inch crêpe pan or nonstick skillet, melt 1 teaspoon butter over medium-high heat.
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Add 1/3 cup chilled crêpe batter and swirl to coat the pan.
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Cook until set on the bottom, about 1 minute; flip and cook for about 30 seconds more.
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Turn out onto a rack to cool.
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Repeat with the butter and remaining batter to make 10 crêpes.
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Place a crêpe on a cake stand or flat plate; spread 1/3 cup pastry cream on top.
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Repeat with the remaining crêpes and pastry cream, ending with a crêpe.
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Refrigerate.
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In a medium saucepan, whisk the chocolate, 3 tablespoons water, the corn syrup and remaining 2 teaspoons butter over medium-low heat until smooth.
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Let cool for 5 minutes; spread on top of the cake, letting it drip down the sides.
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Refrigerate for at least 1 hour or up to 2 days before slicing.