Adjust oven rack to lower-middle position and preheat oven to 325 degrees.
Whisk flour, salt and baking powder in a small bowl. Set aside.
Spray an 8-inch baking pan with cooking spray.
Fit a sheet of foil in pan and up two sides, so you can use it as a handle to pull cooked brownies from pan (Parchment paper/ butter paper can also be used).
Spray sheet with cooking spray.
Melt chocolates and butter in a medium bowl over a pan of simmering water.
Remove from heat, whisk in sugar and vanilla.
Whisk in eggs, one at a time, fully incorporating each one before adding the next.
Continue to whisk until mixture is completely smooth and glossy.
Add dry ingredients, whisk until just incorporated.
Stir in nuts, if using.
Pour batter into prepared pan, bake until a toothpick or cake tester inserted in center comes out with wet crumbs, 35-45 minutes.
Cool brownies in pan on a wire rack for five minutes. Use handles to pull brownies from pan. Completely cool brownies on rack, at least three hours.
Cut into squares and serve. If not serving immediately, do not cut brownies. Whole brownie cake can be wrapped in plastic wrap then foil, and refrigerated for up to five days.
Key Ingredients: All purpose flour, salt, chocolate, butter, egg, walnuts, sugar, condensed milk, baking powder, vanilla essence