Heat the oven to 350ºF. Grease a 12-cup muffin tin and set aside.
In a small bowl, beat together flax egg, pumpkin, milk, syrup, and vinegar.
Add the dry ingredients directly into the bowl with the wet and stir to thoroughly combine. Fold in the chocolate chips.
Fill each cup 3/4 of the way full and let rest for 5 minutes before baking.
Bake on the center rack for 24 - 26 minutes until a cake tester inserted into the center comes out clean.
Cool in the pan for 2 - 3 minutes, then transfer to a wire rack and cool completely. Resist the urge to not break into them; cooling them will allow everything to set properly. If desired, reheat in the toaster oven or in the microwave.