Preheat your oven to 350°F (175°C). Line a 9×13-inch metal baking pan with parchment paper, leaving an overhang on the long sides for easy removal. Lightly grease the parchment and any exposed pan edges.
In a large, heat-safe bowl, add the butter pieces and 1 1/2 cups semi-sweet chocolate chips or chopped chocolate. Microwave in 20–30 second bursts, stirring after each, until mostly melted and smooth, or melt over a double boiler. Let cool for 3–5 minutes until warm but not hot.
1 cup unsalted butter, 1 1/2 cups semi-sweet chocolate chips or chopped chocolate
Whisk the cocoa powder into the warm chocolate-butter mixture until no lumps remain. Add the granulated sugar and brown sugar and whisk until glossy and thick; it may look grainy at first, then like thick chocolate syrup.
1 cup unsweetened cocoa powder, 1 3/4 cups granulated sugar, 1/2 cup light brown sugar
Add the eggs one at a time, whisking well after each addition to build a shiny, crackly top. Whisk in the vanilla extract. The batter should look glossy and slightly thick.
4 large eggs, 2 teaspoons pure vanilla extract
In a separate medium bowl, whisk together the gluten free flour blend, baking powder, salt, and espresso powder (if using) to distribute them evenly.
1 cup 1:1 gluten free all-purpose flour blend, 1/4 teaspoon baking powder, 1/2 teaspoon fine sea salt, 1 teaspoon instant espresso powder
Sprinkle the dry mixture over the chocolate mixture. Using a rubber spatula or wooden spoon, gently fold until just combined and no streaks of dry flour remain. Do not over-mix to keep the brownies tender and fudgy.
Gently fold in the remaining 1 cup chocolate chips or chunks and the chopped nuts, if using. Optionally reserve a small handful of chips or nuts to sprinkle over the top.
1 cup chocolate chips or chunks, 1/2–1 cup chopped walnuts or pecans
Pour the thick brownie batter into the prepared pan and smooth the top with an offset spatula or the back of a spoon. Sprinkle any reserved chips or nuts evenly over the surface.
Bake on the center rack for 24–30 minutes, starting to check around 22 minutes. The brownies are done when the edges are set and a toothpick inserted about 2 inches from the edge comes out with a few moist, fudgy crumbs but no wet batter. The center may look slightly soft; it will firm as it cools.
Let the brownies cool completely in the pan on a wire rack for at least 60–90 minutes. Use the parchment overhang to lift the slab out, then cut into squares with a sharp knife, wiping the blade between cuts for neat edges.