There is also a great book with lots of tips and ideas on how to cook healthy foods for your dog… it’s called The Everything Cooking for Dogs Book by Lisa Fortunato. It has 150 quick and healthy recipes.
edited to add: Moisture is the most common culprit behind moldy biscuits so if you have a convection oven which can maintain the correct baking temp and constantly circulate the heat, the biscuits will dry better. Some recipes will recommend turning off the oven and leaving treats in the oven for a period of time to dry the biscuits/treats out which helps.
Also adding too much fat can cause biscuits and treats to go rancid and improper storage will cause them to spoil.