Heat oven to 350℉. Line an 8x8 inch metal baking pan with foil by placing two long sheets perpendicular to each other in the pan and pressing them flush into the corners and along the sides, leaving the extra hanging over the edges. Lightly grease foil with the extra softened butter, and set the prepared pan aside.
In a small heavy saucepan, melt the chocolate chips, unsweetened chocolate, and butter together over very low heat, stirring constantly. Remove from heat. Then add the unsweetened cocoa and instant espresso powder, whisking until thoroughly incorporated. Set the mixture aside to cool slightly.
In a medium bowl, whisk together the eggs, sugar, vanilla, and salt until well combined.
Add the warm chocolate mixture to the egg mixture and whisk until thoroughly incorporated.
Using a silicone spatula, stir in the flour just until combined. If adding walnuts, gently fold them into the batter.
Transfer batter to the prepared pan, spreading it into the corners with the spatula and then smoothing the top.
Bake until the brownies are slightly puffed, about 35 to 40 minutes.
Allow the brownies to cool in the pan on a wire rack until they've reached room temperature. (About 1½ to 2 hours.) Then remove the brownies from the pan using the foil to lift. Cut into squares and serve.