These tender sweet potato brownies from Zestful Kitchen get their richness from a secret ingredient you probably never saw coming: Sir Kensington's highly versatile Classic Mayonnaise—a balanced blend of smooth sunflower oil , organic free-range egg yolks, and bright hints of lemon. To make this bake really sing, don't skip the crunchy chopped pecans and chocolate chips (because there's no such thing as too much chocolate).
Heat oven to 375ºF with rack set in the middle position.
Line a baking sheet with foil and arrange the sweet potatoes, cut side down, and roast until fork-tender, 35 to 40 minutes.
Remove the sheet pan from the oven, and carefully wrap the foil up and over the sweet potatoes; let steam 5 minutes. Reduce oven temperature to 350ºF and line an 8- × 8-inch baking pan with foil, leaving a 2-inch overhang.
Meanwhile, whisk together maple syrup, nut butter, mayonnaise, and vanilla in a large bowl. Add cocoa powder and whisk until smooth and shiny, about 30 seconds.
Peel the skins from the sweet potatoes and transfer flesh to a medium bowl; mash until smooth. Add 1 cup sweet potato purée, oat flour, salt, and baking powder to the bowl with the nut butter mixture and mix to combine. (Reserve leftover sweet potato puree for another use.)
Spread batter into an even layer in the prepared pan. Sprinkle pecans and chocolate chips over top, if using. Bake brownies until a toothpick inserted in the center comes out clean with a few crumbs, about 30 to 32 minutes.
Transfer the pan to a wire rack and let brownies cool completely in the pan, 45 to 60 minutes. Using the foil edges, lift brownies out of pan and cut into 9 squares.