First, you need to mix together the flour, cornstarch, baking powder, baking soda, and salt together in a bowl until it is well combined. Then, you're going to want to pour this mixture into the mason jar.
Press the flour down firmly into the jar using the back of a spoon. Try to create a nice clean and even line here.
Add in the cup of brown sugar and again press down firmly into jar.
Top with granulated sugar and then the Christmas M&M's.
Seal up jar and create the recipe card for the gift recipient.