In a small bowl, combine the margarine and bacon fat. Mix well and set aside.
In a large bowl, whisk the buttermilk and eggs until well blended.
In another bowl, combine the cornmeal, flour, sugar, cream of tartar, baking soda, and salt. Mix well.
Add the cornmeal mixture to the buttermilk mixture and stir until just blended. Then add the margarine and bacon fat and stir until just blended. Do not overmix. Set aside for 15 minutes. The batter will look slightly thin and will thicken significantly as it sits.
Just before baking, collapse the batter by mixing with a wire whisk. Pour the mixture into a greased muffin pan, filling each cup 3/4 full.
Bake for 20 to 25 minutes, until browned and cooked through. Let cool for 5 minutes before transferring to a wire rack to cool completely.
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Recipe Tips
To make sweet muffins, substitute 8 tablespoons of butter for the margarine and bacon fat. Add an additional 1 tablespoon of sugar to the cornmeal mixture.