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To make the sweet potato purée: Preheat the oven to 400°F with a rack in the center. Using a paring knife or fork, pierce the sweet potato several times. Place on a baking sheet lined with parchment or foil and bake for 45 to 60 minutes, until tender. Carefully slice the baked potato in half lengthwise and set aside until cool enough to handle. Reduce the oven temperature to 350°F.
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To make the topping: In a small bowl, whisk together the flour, sugar, cinnamon, and salt. Using a fork or your fingers, mix in the butter until small crumbs form. Set aside.
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To finish the sweet potato purée: Remove the skin from the sweet potato and transfer 1 cup (240g) of the flesh to the bowl of a food processor. Add the buttermilk and vanilla and process until smooth, about 1 minute. Pause the food processor and scrape the bowl to ensure the ingredients are fully combined. Set the purée aside.
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To make the bread: Lightly grease an 8 1/2" x 4 1/2" loaf pan, then line it with a parchment sling and lightly grease the parchment.
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In a medium bowl, whisk together the flour, cardamom, salt, baking soda, baking powder, and cinnamon.
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In the bowl of a stand mixer fitted with the flat beater or working in a large bowl with an electric hand mixer, beat the butter and sugars on medium speed until fluffy, about 1 minute. Scrape the bowl.
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Add the sweet potato purée and beat on medium speed until combined, about 1 minute. Scrape the bowl. The mixture may look curdled at this point; that’s OK.
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Add the eggs one at a time, beating well after each addition. Scrape the bowl.
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With the mixer on low speed, add the dry ingredients in 2 additions, stopping to scrape the bowl as needed and beating just until combined and a thick batter forms, about 1 minute.
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Scrape the batter into the prepared pan and spread it into an even layer. Sprinkle the topping evenly over the batter.
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Bake the bread for 70 to 80 minutes, or until a skewer or paring knife inserted into the center comes out clean.
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Using the parchment sling, transfer the sweet potato bread to a wire rack and let it cool completely before slicing.
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Storage information: Store sweet potato bread, covered, at room temperature for several days; freeze for longer storage.