Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan or line it with parchment paper for easy removal.
In a large mixing bowl, combine the melted butter and granulated sugar. Mix well until smooth.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In another bowl, whisk together the flour, cocoa powder, baking powder, salt, ground cinnamon, and cayenne pepper (if using).
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Do not overmix.
Fold in the chocolate chips and nuts (if using) until evenly distributed.
Pour the batter into the prepared baking pan and spread it evenly.
Bake for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).
Allow the brownies to cool in the pan for at least 15 minutes before slicing into squares.