Thin Brownies with Pistachios

1/2 cup (1 stick) butter, melted, plus more for pan
1 cup cane sugar
3/4 cup unsweetened cocoa powder
1/2 teaspoon vanilla extract
1/2 teaspoon soy sauce
2 eggs
1/2 cup all-purpose flour
1/2 cup pistachios, coarsely chopped
Optional: flaky salt

1. Place a rack in the center of the oven and preheat oven to 350°F.
2. Generously grease a half sheet pan with butter or shortening. 
3. Mix sugar, cocoa powder, and salt in a medium bowl. Add the butter and stir until smooth. Add eggs one at a time, beating well, until batter is thick, shiny, and smooth. Stir in vanilla. Stir in flour until well incorporated, then continue beating until glossy and smooth.
4. Using a rubber spatula, spread the batter into a thin even layer to about an inch from the edges of the pan. Do not spread all the way to the edges of the pan.
5. Sprinkle the batter with toasted pistachios and a sprinkle of flaky sea salt, if desired.
6. Bake for 10-12 minutes until just firm to the touch. Allow to cool on the pan. Cut brownies into 3-inch squares.