Center a rack in the oven and preheat the oven to 300°F. Line an 8-inch square baking pan with foil, butter the foil, place the pan on a baking sheet, and set aside.
Whisk together the flour and salt.
Put the chocolate in a heatproof bowl and set the bowl over a saucepan of simmering water. Slowly and gently melt the chocolate, stirring occasionally. Remove the bowl from the saucepan and add the butter, stirring so that it melts. It's important that the chocolate and butter not get very hot. However, if the butter is not melting, you can put the bowl back over the still-hot water for a minute. If you've got a couple of little bits of unmelted butter, leave them-it's better to have a few bits than to overheat the whole. Set the chocolate aside for the moment.
Working with a stand mixer with the whisk attachment, or with a hand mixer in a large bowl, beat the eggs and sugar until they are thick and pale, about 2 minutes. Lower the mixer speed and pour in the chocolate-butter, mixing only until it is incorporated-you'll have a thick, creamy batter. Add the dry ingredients and mix at low speed for about 30 seconds-the dry ingredients won't be completely incorporated and that's fine. Finish folding in the dry ingredients by hand with a rubber spatula. (This batter was almost like a mousse when it was done being mixed, and very light in color)
Scrape the batter into the pan and bake 50 to 60 minutes, or until the top is dry and crackled and a knife inserted into the center of the cake comes out clean. Transfer the pan to a rack and allow the brownies to cool to warm or room temperature.
(These brownies ended up with a thin crispy layer on top and rose significantly during baking and then subsequently caved in after cooling slightly!)
Carefully lift the brownies out of the pan, using the foil edges as handles, and transfer to a cutting board. With a long-bladed knife, cut the brownies into 16 squares, each roughly 2 inches on a side, taking care not to cut through the foil.
Serving: The brownies are good just warm or at room temperature; they're even fine cold. I like these with a little something on top or alongside-good go-alongs are whipped crème fraiche or whipped cream, ice cream or chocolate sauce or even all three!
Storing: Wrapped well, these can be kept at room temperature for up to 3 days or frozen for up to 2 months.