Preheat oven to 350 degrees. Line a standard muffin tin with paper liners and set aside.
In a medium bowl, whisk together flour, finely ground tea leaves, baking powder and salt. Set aside.
In a small saucepan, over medium-low heat, bring milk to a simmer. Do not boil. When milk is at a simmer, add tea bags, cover and remove from heat. Let steep for 20 minutes. When done steeping, add 2/3 cup of the milk to a measuring cup.
In the bowl of a stand mixer fitted with a paddle attachment, beat butter and sugar on medium high speed until light and fluffy, about 2-3 minutes. Add egg and mix until well blended. Add vanilla and beat an additional 1 minute.
With a spatula add the flour mixture until just combined, then add the 2/3 cup milk being careful not to over mix.
Spoon batter into liners about 2/3 full. Bake for 18-20 minutes or until a toothpick inserted in the middle of the cupcakes comes out clean. Remove from oven and let cool on wire rack. After cupcakes are totally cooled, frost with vanilla bean buttercream.