Preheat oven to 350°F
Grease or line with parchment a 13" x 9" baking pan. Alternately use an 8.5" x 11" pan. Prepare espresso and let cool to room temperature.
3 Tablespoons brewed espresso, or 2 teaspoon espresso powder
In a large microwave-safe bowl heat butter and chocolate about 2 minutes or until completely melted, stirring halfway through. Or melt in a double boiler.
4 oz dark chocolate, unsweetened, 12 Tablespoons butter, unsalted
Add espresso and vanilla and stir until smooth. Add sugar and mix. Add eggs one at a time and combine completely.
2 teaspoons vanilla extract, 2 cups sugar, 3 large eggs
Add flour and stir to combine, but do not over mix. If desired, add nuts.
1 cup + 2 Tablespoons all-purpose flour
Pour batter into the prepared pan.
Cook for 30-35 minutes or until a toothpick inserted in the center comes out clean or with a few crumbs clinging. If using an 8.5" x 11" pan cook for about 40 minutes. (See Notes below for more tips using an instant-read thermometer)Cool completely before cutting. Store at room temperature in an airtight container for up to 4 days.