Preheat the oven to 400℉ degrees, then line a 13x18 inch baking sheet with parchment paper. Place the sweet potatoes on the baking sheet, and bake for 1 hour.
Once the sweet potatoes have baked for an hour, set them aside to cool completely. Once cooled, remove the skin.
In a large bowl, whisk together the masa harina, sugar, baking powder, cinnamon, nutmeg, ginger and salt. Place the sweet potatoes in the stand mixer bowl, then give them a rough mash to make them easier to work with. Add in the softened butter and molasses, then turn on the speed to low. Alternate between adding in the masa harina ¼ cup (28 g) at a time and the hot water, until everything has been incorporated.
Turn the mixer off, then use a rubber spatula to scrape down the sides and bottom of the bowl. Gradually bring the speed up to medium-low and beat for 30 seconds. Cover the bowl with plastic wrap and let the dough sit for 20 minutes to fully hydrate.
Once the corn husks are rehydrated and pliable, reserve 12 of them for the tamales, then take a couple of the extra husks and rip 24 strips ¼ inch (6 mm) wide and 8 inches long. Tie two together until you have 12 strips 15 to 16 inches (38 to 40 cm) long. You’ll use these to tie the tamales.
To assemble the tamales, grab a corn husk and place it in the palm of your hand. Use a spoon to add about a heaping ⅓ cup (110 g) of the dough to the center of the husk, leaving 1 inch (2.5 cm) free space on the top, left, and right sides and at least 3 to 4 inches (7.5 to 10 cm) of space on the bottom. The dough will expand as it cooks, so if you don’t leave the inch of empty space for clearance at the top, it will ooze out.
Fold the right edge of the husk over the dough toward the left side, then take the left side of the husk and fold it over the right side to create a tube, then fold the bottom flap upward (leaving the top open). Tie a strip around the bottom of the tamal, to secure the bottom fold. As you work, place the folded tamales upright inside a steamer insert, making sure not to crowd the steamer insert.
Add 1½ quarts (1.5 liters) water to a steamer pot. Add the steamer insert, cover, and bring the water to a boil (10 to 15 minutes). Reduce the heat to medium-low and steam for 1 hour 35 minutes to 1 hour 45 minutes. At the 1-hour mark, check the bottom of the pot for water and replenish as needed. If the water for the steam runs out, the bottom of the pot will burn and get ruined. It’s tricky to tell when sweet tamales are done cooking because they’re soft while they’re steaming but firm up once cooled; on average these tamales take about 1 hour 40 minutes to fully cook.
You can start checking for doneness at the 1 hour 25-minute mark. Simply take a tamal out and let it cool for 8 to 10 minutes, then check the inside. If the center looks a little raw or soft, continue to let the tamales steam for another 15 to 20 minutes. Store in an airtight container or gallon-sized resealable plastic bag.
These tamales will keep for 4 days max in the fridge, and 3 to 4 weeks in the freezer. The freshness and flavor starts to decline past this. To reheat, steam for 20 to 30 minutes over medium-low heat until fully warmed through.