In a mixing bowl, cream together the vegetable shortening and milk for about 3 minutes until it resembles small curd cottage cheese. Then, mix in the vanilla extract.

In a separate bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt until well mixed.

Gradually add the dry mixture to the creamed mix in three additions, blending well after each addition.

Gently fold in the egg whites with a spatula until just combined.

Pour the batter into a well-greased and floured 10-inch cast iron skillet. Bake in a preheated oven at 350°F (175°C) for 40-45 minutes or until a toothpick comes out clean.

Allow the cake to cool in the skillet on a wire rack for at least 45 minutes before serving. Dust with confectioner's sugar if desired.