Preheat the oven to 180 C. Butter a square or rectangular cake pan, or line it with parchment paper.
Melt the chocolate and the butter in a bowl in the microwave, stirring occasionally until the mixture is smooth with no remaining lumps.
Add the sugar and mix well with a whisk, wooden spoon or spatula.
Add the eggs and the egg yolk one by one, mixing well after each addition.
Add the flour, baking powder, and salt. Mix well.
Pour the batter into the cake pan, smooth out the surface with a spatula.
Bake for approximately 20 minutes or until the tip of a knife inserted in the center of the brownie comes out clean.
Let cool for 10 minutes before removing the brownie from the cake pan and letting it cool completely on a wire rack.