1Heat the oven to 350°F and arrange a rack in the middle. Coat a 9-by-5-by-3-inch loaf pan with butter and flour; tap out the excess.
2Combine the measured flour, salt, baking soda, baking powder, cinnamon, and nutmeg in a medium bowl and whisk to aerate and break up any lumps; set aside.
3In the bowl of a stand mixer fitted with a paddle attachment, mix the sweet potato flesh, granulated sugar, and brown sugar on medium speed until well combined, about 1 minute. Add the melted butter and mix on low speed until smooth. Add the eggs 1 at a time, mixing until fully incorporated, then mix in the vanilla.
4Stop the mixer and scrape down the sides of the bowl with a rubber spatula. On low speed add half of the reserved flour mixture, then 1/4 cup of the milk. Repeat with the remaining flour mixture and milk, mixing until just combined, about 1 minute. Remove the bowl from the mixer and fold in the nuts.
5Pour the batter into the prepared pan and bake until a cake tester inserted into the center comes out clean, about 55 to 60 minutes. Let the bread cool in the pan for 15 minutes, then turn it out onto a wire rack to cool completely. It will last covered for up to 5 days.