Step 1: Prepare the sweet potatoes: Preheat your oven to 400°F (200°C). Pierce sweet potatoes several times with a fork. Bake for 45-60 minutes or until very tender. Let cool, peel, and mash until smooth. Measure 1.5 cups and let cool completely.
Step 2: In a large mixing bowl, whisk together the cooled mashed sweet potato, granulated sugar, light brown sugar, vegetable oil, eggs, and vanilla extract until well combined and smooth.
Step 3: In a separate medium bowl, whisk together the all-purpose flour, baking soda, salt, ground cinnamon, ground nutmeg, ground ginger, and ground allspice.
Step 4: Gradually add the dry ingredient mixture to the wet ingredient mixture, mixing until just combined. Be careful not to overmix.
Step 5: Preheat your oven to 350°F (175°C). Grease and flour a 9×5 inch loaf pan, or line it with parchment paper.
Step 6: Pour the batter evenly into the prepared loaf pan.
Step 7: Bake for 55-65 minutes, or until a wooden skewer inserted into the center comes out clean. The top should be golden brown and spring back when lightly touched.
Step 8: Remove the loaf from the oven and let it cool in the pan for 10-15 minutes before inverting it onto a wire rack to cool completely. Slice and serve.