Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper.
In a medium saucepan set over low heat, melt the butter and chocolate, stirring frequently.
When melted and smooth, remove from heat and stir in the brown sugar. Add the eggs one at a time, mixing after each.
Stir in the vanilla and espresso, if using.
Gently stir in the flour and salt, stirring only until streaks of flour are almost gone. Stir in nuts, saving some to sprinkle on top.
Pour batter into the prepared baking pan. Top with remaining nuts.
Bake until edges are firm and dry, and center is set but not dry, about 35-40 minutes. Sprinkle with additional salt, if desired.
Let cool completely before slicing.
Calories: 391kcal, Carbohydrates: 36g, Protein: 5g, Fat: 26g, Saturated Fat: 12g, Cholesterol: 72mg, Sodium: 98mg, Potassium: 204mg, Fiber: 2g, Sugar: 26g, Vitamin A: 422IU, Vitamin C: 1mg, Calcium: 48mg, Iron: 2mg