In a small bowl, add cornstarch, baking soda and cream of tartar. Whisk until fully combined.
Mixture is ready to use immediately.
Notes
Store homemade baking powder in a glass jar or storage container.
Storing it in the refrigerator is best.
Check for activeness by adding ½ teaspoon to ¼ cup hot water. If it fizzes, it's fine to use, if not, then it is no longer fresh.
DIY baking powder is similar to single acting baking powder and will need to be baked immediately. Keep this in mind as most gluten free batters need a rest time before baking.