Prepare the Pan: Preheat the oven to 350°F. Grease an 8x8-inch square pan or 9x5-inch loaf pan, or line it with parchment paper or foil. Set aside.
Melt the Butter: Place the butter in a large microwave-safe mixing bowl and microwave for about 1 minute, or until completely melted. If using oil instead of butter, add it directly to the bowl.
Combine the Sugar and Cocoa: Add the sugar and unsweetened cocoa powder to the melted butter or oil. Whisk vigorously for at least 1 minute, until the mixture is well combined and the sugar has dissolved.
Add the Eggs and Vanilla: Add the eggs one at a time, whisking them into the mixture. Add the vanilla, then continue whisking for at least 1 minute, until the batter is well combined and shiny.
Mix in the Dry Ingredients: Add the flour and salt, then stir just until the flour is fully incorporated. Do not overmix the batter.
Fold in Optional Add-Ins: If using, gently fold in up to 1/2 cup of nuts, raisins, chocolate chips, chopped dark chocolate, or another preferred add-in.
Fill the Pan: Spread the brownie batter evenly into the prepared pan.
Bake the Brownies: Bake for approximately 20 minutes, or until the center is slightly set. Avoid overbaking, as this can make the brownies dry and cakey.
Cool and Slice: Allow the brownies to cool completely before cutting. Slice into 9 large squares or 16 smaller brownies.
