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.
In the bowl of a stand mixer combine butter and sugar. Beat on low speed until blended, about one minute. Add the vanilla bean seeds and heavy cream, starting with two tablespoons, and increase speed to medium and continue to beat for 4 minutes until smooth and creamy. Add more heavy cream if needed to get to desired spreading consistency.