Preheat the oven to 350 degrees F. Line two muffin tins with liners and set aside.
In a medium-sized bowl, whisk together the flour, baking powder, and salt. Set aside.
In the bowl of a stand mixer, beat the butter for a few minutes until it is smooth. Add the granulated sugar, and cream together with the butter for 5 minutes, or until light and fluffy.
Add the eggs 1 at a time, making sure to mix well after each addition. Scrape the sides of the bowl as needed.
In two batches, add the dry ingredients to the butter/sugar/egg mixture. Add the sour cream, lemon juice, vegetable oil, lemon oil/zest, and vanilla extract.
Fill the muffin tins ⅔ of the way. Bake 22-25 minutes, or until a toothpick/knife comes out clean when inserted in the center. Allow the cupcakes to cool in the pans for 5 minutes before removing them to a wire rack to finish cooling completely.
To make the buttercream
Boil the ½ cup of water and add the 4 tea bags. Allow them to steep for 45 minutes. Squeeze all of the liquid of of the tea bags before removing and discarding them. Cool the tea in the refrigerator for at least 30 minutes.
Beat the butter for 2 minutes before adding 2 cups of powdered sugar. Beat until incorporated. Add an additional two cups of powdered sugar, beating until combined again.
Add the lemon extract and then slowly stream in the cooled tea. Add the remaining two cups of powdered sugar until the frosting reaches the desired stiffness. Beat for a few minutes on medium-high speed until it is light and fluffy.
Once the cupcakes have cooled completely, frost them however you'd like. Top with lemon slices and mint leaves.
Notes
Notes: This recipe will likely yield more buttercream than you actually need for 2 dozen cupcakes