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. Line the bottom of each with a round of parchment paper, and then butter the paper.
Sift the flour, cocoa, baking soda and salt together onto parchment paper. With an electric mixer on medium-high speed, cream the butter and sugars until light and fluffy, 4 or 5 minutes. Add your eggs, 1 at a time, mixing until completely incorporated before adding each additional egg. Mix in the chocolate and vanilla, making sure to combine really well. 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. Make sure to let it mix thoroughly.
Pour half the batter in each pan and spread it evenly with a spatula or butter knife. Bake until a toothpick comes out clean, 25 minutes. Remove the pans to racks and allow the cakes to cool for 10 minutes or so. Loosen the edges of the cake from the pan with a knife, then turn them onto the racks. Remove the paper and let the cakes completely cool.