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1
Preheat oven to 350u00b0F (176u00b0C). Butter two 9-inch round baking pans, line with parchment paper and butter the parchment. Set aside.
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2
Sift plain flour and cake flour into a mixing bowl. Add baking powder, baking soda and salt and whisk 20 seconds, set aside.
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3
In the bowl of an electric stand mixer fitted with the paddle attachment (or use a hand mixer and a large mixing bowl), whip together butter and granulated sugar until pale and fluffy. Mix in vegetable oil, then eggs one at a time, and then egg yolks. Mix in vanilla. Add 1/3 of the flour mixture and mix just until combined, then add in 1/2 of the buttermilk and blend until combined. Repeat process once more then finish by mixing in remaining 1/3 of the flour mixture.
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4
Divide batter among prepared cake pans. Bake for 25-29 minutes until toothpick inserted into centre comes out clean. (If baking cakes one at a time, bake for about 20 minutes.)
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5
Cool in pans for several minutes. Run a knife around the edge to make sure they are loose, then invert onto wire racks to cool completely. Frost with chocolate buttercream. Store in an airtight container.
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6
For the buttercream, in the bowl of an electric stand mixer fitted with the paddle attachment, whip butter on moderately high speed until very pale and fluffy. Sift in cocoa powder then add icing sugar, single cream, and vanilla. Mix on low speed until combined, using up to an additional 2 tablespoons of single cream to thin as needed. Increase mixer to medium-high speed and whip until fluffy.
