An hour or two before you plan to bake, set out your butter and eggs to come to room temperature.
Preheat oven to 350F. Line a muffin pan with cupcake liners, and lightly spray each liner. This keeps them from sticking, since the actual cakes portion doesn't contain fat or butter.
In a small microwavable bowl or measuring cup, heat your milk until steaming, about 30 seconds. Add your teabags and let steep for at least 5 minutes.
While that is steeping, mix together your flour, cornstarch, baking powder, and salt in small bowl. Set aside.
Remove your teabags (squeeze well) and add your vinegar to the tea-milk and mix - it will look curdled because this is substitute for buttermilk. Open one of your tea bags and stir those leaves into your dry ingredients for the best earl grey flavor!
Cream your butter and sugar for 1-2 minutes. Add egg and vanilla and beat for another minute, until very well mixed.
Add half of your tea-milk alternatively with half of your flour mixture. Beat at medium speed after each addition. The batter will be pretty thick!
Optional step: Once everything is combined, I recommend letting the batter rest for at least 30 minutes. If you want to let it rest a few hours or overnight, cover and place in the fridge. This resting helps prevent graininess in GF baking by giving the flours time to absorb moisture before baking.
Scoop the batter into the lined cupcake pan, and tap it on the counter a few times to smooth it out. Bake 18-20 minute or until a toothpick inserted in the center comes out clean.
Let cool in pan for a minute, then transfer onto a wire rack to cool completely.