So the world is divided between those who like their brownies cakey, and those who like them gooey. I'm in the latter camp, and this is the recipe (after about half a dozen) that I settled on a few years ago, as my best. It comes from Cook's Illustrated, and I've abandoned all other recipes in favor of this one. If you like this recipe, see others at my website: mybestcookbook.wordpress.com
Preheat oven to 325. Take foil and line a 13x9 baking pan with foil (large enough to overhang so you can lift out the brownies. Grease the foil.
Whisk together the flour, baking powder and salt. Melt the butter and chocolate in a medium bowl, (heatproof) over simmering water (or microwave)until melted. Stir often to combine. Whisk in vanilla.
Whisk in the sugar. Add eggs one at a time, whisking in thoroughly. Then add flour in three additions, folding in with spatula, until batter is smooth.
Move batter into the baking pan, even out the surface and sprinkle with the nuts. Bake approx 30-35 min. or until skewer comes out with a few crumbs attached. Cool for two hours before removing with the foil sling and cutting.