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Preheat oven to 350°F/177°C. Grease and line a 9-inch square pan. Set aside.
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Mix party cupcake mix according to package instructions. Pour cake batter into prepared pan. Bake in the preheated oven for 20-25 minutes or until set.
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Set aside cake to cool.
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In a nonstick pan, mix together yolks, evaporated milk, condensed milk, all-purpose cream and vanilla. Heat over low fire until mixture thickens.
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Once the mixture thickens, add in butter. Pour into a container, cover and refrigerate.
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Cut square cake in half. On one piece, spread filling, top with other half of the cake.
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Refrigerate to set.
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Cut into desired portions. Serve.