In a medium bowl, whisk together the flour, baking powder and salt.
Put the butter into a medium heavy-bottomed saucepan over medium-low heat.
When the butter is partially melted, add the chocolate.
Stir occasionally until the chocolate is melted and the liquid is smooth.
Remove the pan from the heat and whisk in the sugar. The batter will look granular.
Add the eggs one at a time, whisking well after each.
Whisk in the vanilla. The batter will be smooth and shiny.
Add the dry ingredients and stir them in with a large rubber spatula. When not quite incorporated, add the chocolate chunks and stir only until the dry ingredients and chocolate are thoroughly mixed in.
Spread the batter in the prepared pan and place the pan in the oven.
Bake 25 minutes.
A toothpick inserted about 2 inches into the edge of the brownie “cake” should come out clean, but a toothpick thrust into the center of the brownie should come out with moist chocolate sticking to it. Bake another 5 minutes or so if necessary, but do not overbake. Note the sugary crust.
Cool completely on a wire rack.
Cool completely on a wire rack.
Cover tightly with foil and let stand overnight.
To serve, cut into squares with a sharp knife.