Sift the flour, sugar, cocoa, baking soda, cream of tartar, and salt into a mixing bowl.
Add the shortening and 2/3 cup whole milk to the sifted mixture and beat until smooth.
Add the eggs, 1/3 cup whole milk, and vanilla and beat until mixed.
Pour the batter into a 9x13-inch baking dish and bake for 30 to 40 minutes.
Caramel Frosting
Combine the butter, brown sugar, and salt in a small saucepan over medium heat and stir until the butter has melted. Raise the heat and bring the mixture to a hard boil for 2 minutes, stirring constantly.
Remove the saucepan from the heat and add the milk, stirring vigorously.
Return the saucepan to the heat and bring to a full boil again. Set aside to cool until lukewarm.
Add the powdered sugar and stir until smooth.
When the cake has cooled, frost it with the caramel frosting.
Recipe Tips
This recipe comes from Sue A. Mertens Minick from Hazelwood, MO. She found this site on the internet! And yes, she is family, she is a family member of mine.