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Grease a Pullman pan or a 9" x 5" loaf pan. For best results, line the pan with a parchment sling and lightly grease the parchment. (The sling will help you remove the delicate loaf from the pan.)
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Preheat the oven to 350°F.
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In a large heatproof bowl, combine the butter, sugar, and salt. Microwave on high for 1 1/2 to 2 minutes, until the butter is melted. Remove from the microwave, add the cocoa, and stir the mixture thoroughly. (Alternatively, melt the butter, sugar, and salt in a large saucepan over medium-low heat. Remove from heat and add the cocoa.)
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Using a wooden spoon or a firm rubber spatula, mix in the eggs one at a time, stirring well after each addition. The mixture will go from grainy to glossy as you vigorously stir. Stir in the vanilla.
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Add the flour and baking powder, stirring until evenly combined. Scrape the sides of the bowl to make sure there are no dry floury spots.
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Make sure the batter is cooled to lukewarm, then stir in the chips and walnuts.
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Transfer the batter to the prepared pan, place on a baking sheet, and bake for 48 to 50 minutes. When it's done, the brownie loaf will have risen slightly and a paring knife inserted into the center should come out mostly clean, with a few moist crumbs sticking to it.
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Remove the pan from the oven and cool on a wire rack for at least 1 1/2 hours. Lift the brownies out of the pan using the parchment sling (or gently flip it out if no parchment was used). Slice the brownie loaf into 1/2"-thick slices and serve with a scoop of ice cream and a drizzle of caramel sauce on top, if desired.
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Store any leftover brownie slices, well-wrapped, at room temperature for 5 to 6 days. Freeze for longer storage.
