Preheat your oven to 350°F (175°C) and lightly grease an 8x8 inch baking pan. In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, and salt until well combined.

In a separate bowl, combine the melted butter, eggs, vanilla extract, and milk. Pour the wet ingredients into the dry ingredients and mix until just combined, being careful not to overmix. Stir in any optional chocolate chips or nuts.

Pour the batter into the prepared baking pan and spread evenly. Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with moist crumbs. Let cool completely before cutting into squares and serving.