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Grease a ridged cake loaf pan, a Pullman pan, or a 9" by 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 40 to 43 minutes for the ridged cake loaf pan, or 48 to 50 minutes for the Pullman or loaf pan. 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 90 minutes. 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.
