Feliz cumpleaños, Jorge.
1/2 cup unsalted butter, softened
1 cup firmly packed light brown sugar
1/2 cup granulated sugar
1 large egg, at room temperature
1 cup buttermilk
1 teaspoon vanilla extract
1 1/2 cups flour
3/4 cup cocoa powder
2 tablespoons espresso powder
1/2 teaspoon baking soda
1/4 teaspoon salt
1/2 cup melted chocolate (whatever leftover chips or scraps you may have)
Preheat the oven to 325°F. Butter and lightly flour a 9x5x3-inch loaf pan. In a large bowl, on the medium speed of an electric mixer, cream the butter until smooth. Add the sugars and beat until fluffy. Add the egg and beat well, then the buttermilk and vanilla. Sift the flour, cocoa, baking soda, espresso powder and salt together right into your wet ingredients. Stir together with a spoon until well-blended but do not overmix. Fold in melted chocolate. Scrape down the batter in the bowl to make sure everything is mixed in.
Pour the batter into loaf pan and bake for 70 minutes, or until toothpick comes out clean. Cool in pan on a rack for about 10 to 15 minutes before you turn out.
I got the recipe here.