Place the lard or vegetable shortening and 1/2 teaspoon of salt into a mixer, and beat over medium speed until very light, about 1 minute. Reduce speed to low, add baking powder and sugar, and take turns adding the corn masa flour and the broth. Raise speed back to medium and continue beating another 6 to 7 minutes, until the dough is homogeneous. In batches, add the cooled sweet potato pulp and continue beating for another 5 to 6 minutes, until the masa looks fluffed up.