Preheat the oven to 350°F (175°C). Grease and flour two 8-inch (20-cm) cake pans. It's not a problem if you only have one. Get it ready.
In a bowl, sift together the flour, cocoa, baking powder, baking soda, and salt. Set aside.
In the bowl of a mixer, beat the oil and sugar together (for about 2 minutes), until combined. With the mixer running on low, add the eggs, egg yolk, and vanilla extract. Stop the mixer and scrape down the bowl.
With the mixer on low, add the flour and cocoa powder mixture in three batches, alternating with the milk. Begin and end with the flour mixture. Stop and scrape down the bowl once again. With the mixer running on low, add the coffee and mix until combined and you get a smooth batter.
Divide the batter between two prepared pans, or spoon a half of the batter into the pan, if using one. Bake for 25-27 minutes, or until a toothpick comes out clean. Let cool for 15 minutes before removing from the pan.