Preheat oven to 350°F (175°C).
Line a 9x13 inch baking pan with parchment paper.
Whisk together melted butter and sugar until smooth.
Add in the eggs, one at a time, whisking after each addition.
Stir in vanilla extract and brewed espresso (if using).
In a separate bowl, whisk together flour, cocoa powder, espresso powder, salt, and cinnamon (if using).
Gently fold dry ingredients into the wet mixture using a rubber spatula.
Stir in chocolate chips, walnuts, and espresso extract (if using).
Pour batter into prepared pan and smooth the top.
Bake for 25–30 minutes, or until a toothpick inserted comes out with moist crumbs.
Let brownies cool completely in the pan on a wire rack, about 2 hours.
Cut into 16 squares and serve.