Preheat the oven to 450 degrees F.
In a small skillet, cook the bacon over medium heat until crisp, 5 to 7 minutes. Remove and drain on paper towels until cool, then chop into small pieces. Add the onion to the skillet with the bacon grease and cook over medium-low heat until light golden, about 5 minutes. Drain on paper towels.
Stir together the flour, powdered milk, baking powder, sugar, cream of tartar and salt in a medium bowl. Using a pastry cutter, cut the chilled butter bits and shortening into the dry mixture until blended thoroughly. Stir in the bacon and onion. Fold in the buttermilk until the dough comes together.
On a lightly floured surface, turn out the dough and use your hands to press into a circle 1 to 1 1/2 inches thick. Cut out circles with a biscuit cutter or a glass. Place the biscuits in cast-iron skillets, then brush the tops with the melted butter. Bake until golden brown, about 15 minutes.