You can use room temp butter or oil, or a combination of both. I recommend using canola oil since it will provide neutral flavor to the cupcake.
Buttermilk yields a light, tender crumb. If you don't have buttermilk on hand, you can easily make it by placing 1 T. of vinegar into a liquid measuring cup and add whole milk up to make it 1 c.
If time allows, be sure to let the batter rest as hydrating the gluten-free flour is an important step for light and tender bakes. You can make the batter ahead of time and place in the fridge and scoop batter when needed.
For extra light and fluffy frosting, add 1 egg white or 1 T. meringue powder.
Storage: Gluten-free baked goods do not have the same shelf-life as conventional baked goods. Store in airtight container on counter 1-2 days, refrigerate 3-5 days, or seal in plastic and place in freezer safe container for up to 6 months. When freezing, allow to come to room temperature before opening the container.