Rehydrate masa harina. Bring the broth to a boil. Place masa harina in a large heat-proof bowl and pour 6 cups of the broth over the masa harina. Stir until masa is evenly moist. If it is still really dry add another cup of broth. Save the final cup for adding later.
Beat fat. Place vegetable shortening (or which ever fat you are using) in the bowl of a stand mixer fitted with a paddle attachment. Beat on high until light and fluffy, about a minute.
Add remaining ingredients. Add baking powder and salt and continue beating until light and fluffy, about a minute more. Turn off the mixer and scrape down the sides of the bowl with a rubber spatula.
Slowly add in masa. Turn mixer back on a put on medium speed. Start adding masa dough a handful at a time, letting it whip in before adding the next. You want to really beat the dough and incorporate a lot of air. Keep adding the masa until all the dough is added.
Add more broth if necessary. Stop the mixer and feel the dough, it should be light and fluffy like whipped mashed potatoes, maybe even a bit wetter than that. If it is still too dry, add more broth, 1/2 cup at a time until it is almost soupy. It will dry as it sits so it is better to be on the wet side.
Fill and assemble tamales. Use dough to fill and assemble your tamales.