Heat oven to 350°.
Sift flour and salt together; set aside.
*Optional step for peanut butter filling: With electric mixer, beat peanut butter, powdered sugar, and butter until very creamy, about 3 minutes. Set aside.
To make the brownies, melt chocolate with butter in double boiler, stirring frequently. Alternatively, you can melt in microwave in 30-second increments, stirring in between. After dissolving, add 1 cup of the sugar to the mixture; stir to blend. Add vanilla and stir.
Pour the mixture into a large bowl.
In stand mixing bowl whisk together the remaining cup of sugar and the eggs until just combined.
Little by little, pour half of the sugar and eggs into the chocolate mixture, stirring gently but constantly with a rubber spatula so that the eggs don't set from the heat.
Using whisk attachment, whip the remaining sugar and eggs until they are thick, pale, and doubled in volume, about 3-4 minutes.
Using rubber spatula, delicately fold the whipped eggs into the chocolate mixture.
When the eggs are almost completely incorporated, gently fold in the dry ingredients.
Pour and scrape the batter in to an unbuttered 9-inch square pan. Note: I sprayed with a little baking spray just to make sure they didn't stick.
*Optional step: drop teaspoonfuls of peanut mixture on top of the batter and throughout the pan.
Bake the brownies for a minimum of 45 minutes, during which time they will rise a little and the top will turn dark and dry.
Cut into the center at about the 30-minute mark to see how the brownies are progressing: they are perfect when they are just barely set and still a little gooey. Note: Some people like them a little less gooey. If so, bake an extra few minutes.
Cool the brownies in the pan on a rack.
Optional: sprinkle with powdered sugar.