Combine flour, baking soda, salt, and oats in a large bowl.
In a separate bowl, cream starter, butter, shortening, and sugars. Add eggs one at a time, then mashed banana and vanilla. Mix well.
Create a well in the dry ingredients and add wet ingredients, stirring until incorporated.
Fold in whatever mix-ins you'd like: chocolate chips, shredded coconuts, nuts, etc.
Drop rounded tablespoons onto an ungreased baking sheet (or use parchment paper for easy clean-up).
Bake 12 to 14 minutes, or until cookies begin to brown on the edges.
ENJOY!
Notes
Make this in advance: store dough (well-wrapped) in the fridge up to two days before baking. If you'd like to reserve some for the future, wrap well and freeze.