Sift flour, baking soda and salt together. Cream shortening and sugar until light and fluffy. Add egg and beat well. Add chocolate, peppermint oil and vanilla and blend thoroughly. Beat in yogurt.
Add flour mixture and milk alternately in small amounts. Beat well after each addition. Pour batter into two well-greased 9-in. pans or one 9" X 13" pan. Bake at 350 degrees for 30 to 40 minutes or until cake tests done.