Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan. In a small bowl, mix together the cocoa and hot water and set aside.
In a large bowl, beat together the sugar and shortening until smooth. Beat in the egg and stir in the vanilla. Sift together the flour, baking soda and salt. Beat in the flour mixture alternately with the sour milk. Stir in the cocoa and water mixture. Pour batter into prepared pan.
Bake in the preheated oven for 35 to 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.