Foods sold under the Cottage Food Law must be non-potentially hazardous.
The definition of a non-potentially hazardous food is on pages 11-13 of the Texas Food Establishment Rules.
There is a chart that you can read to determine whether the combination of pH and water activity (aW) makes that item potentially hazardous or not.
If you have a frosting or filling recipe that you would like to have tested, there is a company in Texas called Food Safety Net Services
which will test a sample of your food for pH and aW for around $32.00. There is no setup fee for Texas customers. They have offices in Dallas and San Antonio, and you can either bring in your sample or overnight it on ice.
This is a great resource for Texas home bakers who are wondering if their recipe is allowed under the law.