Preheat the oven to 180C/350F.
Bring milk to a boil and add 4 bags of Earl Grey tea, let them steep for 5 minutes, then remove the bags and let the milk cool, then strain though a fine sieve.
Grease 2 8"/20cm cake tins with some butter and sprinkle them with flour, set aside.
In a medium bowl sift together flour, baking powder and salt.
In the bowl of a stand mixer beat the softened butter until light and fluffy with a paddle attachment, add sugar and keep mixing until incorporated, about 3 minutes, then add egg yolks one at a time, giving them time to be completely mixed in before each addition, then add vanilla.
While the motor of the stand mixer still running add half the flour mixture, then half of the tea infused milk, then the remainder of the flour mixture and finish with the second half of tea infused milk. Stop the mixer and scrape down the sides with a spatula, add zest of one lemon and mix for 1 more minute.
Divide the batter among two prepared pans and bake in the oven for 25-30 minutes or until a cake tester inserted in cakes comes out clean.
Remove on a wire rack to cool, when cool to the touch remove from the pans and cool completely on a wire rack.