Step 1In the bowl of a heavy-duty mixer, combine the butter and margarine. With the paddle attachment on high speed, whip for 2 minutes, or until fluffy. Lower the speed to medium and add the fresh masa alternately with 1 cup of the stock, then add the salt. Beat until well mixed.
Step 2Return the mixer to high speed and whip for 3 to 5 minutes, until the masa is the consistency of spackle. If necessary, add more stock, one-fourth cup at a time, until the correct consistency is attained.
Step 3Drop one-half teaspoon masa into a cup of cold water. If the masa floats, it is ready; if it sinks, continue whipping it for another minute. Repeat this "float test" until the sample masa floats.