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Preheat the oven to 350 degrees and spray a 9x13 glass baking pan with non-stick spray. Set aside.
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Whisk the buttermilk, eggs, egg yolks, and vanilla in a large bowl until well combined.
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Whisk the flour, sugar, baking soda, baking powder, and salt in a large mixing bowl.
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Add the oil and softened butter and mix with a hand mixer until well combined. Then, add the egg and buttermilk mixture.
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Mix with a hand mixer, the color lightens, and it becomes fluffy.
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Pour the batter into the pan and bake for 26 to 30 minutes until a toothpick inserted into the center comes out clean or with dry crumbs.
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Remove the pan and place it on a wire rack. Cool completely.
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Melt the dark chocolate chips or chopped dark chocolate in the microwave at 20-second intervals until melted and smooth.
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Add all the frosting ingredients except the melted chocolate to a large mixing bowl.
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Mix with a hand mixer on low speed for about a minute. Then, turn the speed to medium and mix for another minute until fully combined.
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Mix the melted chocolate with a whisk until shiny, then add to the frosting and mix again for another 30 seconds until smooth.
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Cover the bowl with parchment paper and let it set for about 35 minutes.
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When the cake is thoroughly cooled, top it with the fudge frosting and enjoy!