Preheat your oven to 350°F (175°C). Grease a 9x9 inch baking pan.

In a microwave-safe bowl or on the stove, melt the butter.

In a large mixing bowl, combine melted butter and sugar. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.

Fold in chocolate chips or chunks and chopped nuts if desired.

Pour the batter into the prepared baking pan and spread it evenly. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).

Allow the brownie cake to cool in the pan for about 1-2 hours, then transfer it to a wire rack to cool completely.

Once cooled, cut into squares or rectangles, and enjoy your delicious brownie cake!