Preheat the oven to 350 degrees. Grease and flour a 13 x 9 x 2-inch cake pan.
Sift the flour with the salt. Melt the chocolate with the shortening over hot water; turn into a large mixing bowl and cool to room temperature. Add the sugar and mix well at low speed. Blend in the egg yolks and 1 cup milk. Add the flour mixture; mix until all the flour is dampened. Then, with the mixer at low speed, beat 1 minute. Add the vanilla and ½ cup milk; mix until smooth. Dissolve the baking soda in the remaining ¼ cup milk; stir quickly but thoroughly into the batter. Turn in pan. Bake for 45 minutes, or until a cake tester inserted in the middle of the cake comes out clean. Either leave in the pan to cool and frost, or cool in the pan for about 10 minutes and then turn out to cool completely.
Source: “Good Housekeeping’s Cake Book”, Consolidated Book Publishers, 1958