Cream together the sugar, butter and shortening until smooth then beat in vanilla. Add eggs, one at a time, until fluffy.
In a separate bowl, sift together the dry ingredients (flour, baking powder, and salt). Add this flour mixture and the evaporated milk to the original mixture alternately. Beat for 2 minutes on medium speed, then fold in the sour cream.
Spread onto a parchment lined half sheet pan. Bake at 325°F until a toothpick inserted in the middle comes out clean (25-35 minutes – but check at 25 minutes!).