Preheat the oven to 425 F (218 C). Line a baking sheet with parchment paper. Clean and dry 2 - 4 medium sweet potatoes. Pierce the sweet potatoes 5 - 6 times all around the potato. Roast for 40 - 45 minutes or until fork tender. Cool to room temperature, scoop out the sweet potato flesh into a food processor. Blend until smooth. Measure out 1 ¼ cup (300 grams) of pureed sweet potato. Alternatively, use canned sweet potato. In a mixing bowl, whisk together the flour, granulated sugar and ground cinnamon for the streusel topping. Work in the room temperature butter a little at a time until a sand like texture develops. There will be a few bigger crumbs, but most should be finer, smaller pieces. Place the bowl in the fridge until the sweet potato loaf is ready for topping.
⅓ cup (40 g) all purpose flour, 2 tablespoons (25 g) granulated sugar, ¼ teaspoons ground cinnamon , 2 tablespoons (28 g) unsalted butter, room temperature
If the oven is still on from roasting the sweet potatoes, reduce the temperature to 350 F (177 C). If the oven has been turned off, then preheat the oven to 350 F for about 20 - 30 minutes. Line a 9 x 5 (or 8.5 x 4.5) loaf pan with parchment paper, leaving a bit of overhang on two sides.
In a large bowl, whisk the flour, baking soda, ground cinnamon, ground ginger, ground nutmeg, and salt together until well combined.
1¾ cup (210 g) all purpose flour, 1 teaspoon baking soda, 2 teaspoons ground cinnamon , 1 teaspoon ground ginger, ½ teaspoon ground nutmeg, ½ teaspoon salt
In another large mixing bowl, whisk the oil and light brown sugar until well combined. Whisk in the eggs, sweet potato puree, buttermilk and vanilla extract until smooth and well combined. Whisk the dry ingredients into the wet until mostly smooth. A few small lumps are fine. Pour the quick bread batter into the prepared loaf pan. Top the sweet potato loaf bread batter with the streusel (if using). ½ cup (120 ml) neutral baking oil or melted butter, 1 cup (200 g) light or dark brown sugar, 1¼ cups (300 g) pureed sweet potato, 2 large eggs, room temperature, ¼ cup (60 ml) buttermilk, room temperature, 2 teaspoons (10 ml) vanilla extract
Bake the sweet potato bread for 45 - 55 minutes for a 9 x 5 (or about 10 minutes longer if using a 8.5 x 4.5 loaf pan) or until a toothpick inserted into the center of the loaf comes out with a few moist crumbs. The edges of the bread will also pull slightly away from the edges of the pan.
Cool the loaf cake in the loaf pan for 15 minutes. Use the parchment sling to lift the bread out of the pan. Allow the sweet potato bread to full cool at room temperature for about an hour before slicing or topping with a glaze or frosting.