Preheat the oven:
Preheat the oven to 175°C. Prepare a square pan (approximately 20 x 20 cm) and grease it with butter or line it with baking paper.
Melt the butter:
Melt the butter in a small saucepan or in the microwave, then pour it into a bowl.
Add the ingredients:
Add the sugar and mix well, then add the eggs and vanilla and stir until combined.
Add the dry ingredients:
Add the cocoa, flour, baking powder, and salt. Mix until you get a thick batter.
Add the chocolate (optional):
Add the chocolate pieces and stir gently.
Bake:
Pour the batter into the pan and spread it out, then bake for 20–25 minutes. Test with a wooden skewer: it should come out with a few crumbs, not completely clean (so the brownies are soft inside).
Cool and cut:
Let it cool completely before cutting into squares.