Preheat your oven to 350°F (175°C) to ensure it's hot when you put the brownies in.
Grease an 8x8 inch baking pan with butter or line it with parchment paper for easy removal.
In a large mixing bowl, sift together the all-purpose flour, cocoa powder, granulated sugar, brown sugar, baking powder, and salt until well combined.
In a microwave-safe bowl, melt the unsalted butter in the microwave or over low heat until liquid but not bubbling.
In a separate bowl, whisk together the melted butter, eggs, vanilla extract, and milk until smooth.
Pour the wet ingredients into the bowl with the dry ingredients and gently fold them together using a spatula until just combined. Do not overmix.
If using, fold in the chocolate chips and chopped nuts until evenly distributed.
Pour the brownie batter into the prepared baking pan and spread it evenly with a spatula.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, remove the brownies from the oven and let them cool in the pan for 10 minutes before transferring to a wire rack to cool completely.