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1
Put a rack in the middle of the oven and preheat the oven to 350 degrees F. Butter 2 (9 by 2-inch) nonstick round cake pans.
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2
Line the bottom of each with a round of parchment paper, and then butter the paper.
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3
Sift the flour, cocoa, baking soda and salt together onto parchment paper.
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4
With an electric mixer on medium-high speed, cream the butter and sugars until light and fluffy, 4 or 5 minutes.
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5
Add your eggs, 1 at a time, mixing until completely incorporated before adding each additional egg.
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6
Mix in the chocolate and vanilla, making sure to combine really well.
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7
Turn the mixer to low and slowly add the dry ingredients, alternating with the buttermilk, in 3 batches, starting and ending with the flour mixture.
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8
Make sure to let it mix thoroughly.
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9
Pour half the batter in each pan and spread it evenly with a spatula or butter knife.
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10
Bake until a toothpick comes out clean, 25 minutes.
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11
Remove the pans to racks and allow the cakes to cool for 10 minutes or so.
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12
Loosen the edges of the cake from the pan with a knife, then turn them onto the racks.
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13
Remove the paper and let the cakes completely cool.
