For the earl grey tea leaves, use a spice grinder or food processor to break up the large leaves and coarsely grind into smaller pieces (not powder). If not, you can also put the leaves in a Ziploc bag. Zip the bag and hit with a rolling pin to break them into smaller pieces.
Place the oven rack to lower-middle position. Preheat the oven to 350˚F (177˚C). Grease a loaf pan (9x5-inch) thoroughly with butter.
In a large mixing bowl with an electric mixer fitted with a paddle attachment, beat together butter, sugar, baking powder and salt until very pale, light and fluffy, about 8 minutes. Scrap down the paddle and bowl. With mixer on medium-high speed, add eggs and egg yolk one at a time, scrapping down the paddle and bowl after each time.
In a small bowl, whisk together sour cream, lemon zest and vanilla extract until smooth.
In a medium bowl, combine flour and tea leaves.
Back to the mixer, add half of the sour cream into the batter. Mix on medium-low speed, about 15 seconds. Add half of the flour and mix for another 15 seconds. Add the rest of the sour cream and mix for 15 seconds. Add the flour and mix again 15 more seconds. Finish mixing the batter with a spatula to make sure everything is incorporated.
Transfer the batter to the prepared loaf pan and roughly smooth the top. Swirl the batter with a butter knife or offset spatula to remove air pockets. Bang the loaf pan on the counter a couple times to remove air pockets. Smooth the top once again. Wash a butter knife under tap water to wet both sides, then run it into the center of the loaf cake lengthwise (about 1-inch deep into the cake), which will help to create a beautiful crack in the middle.
Bake in the oven until dark golden brown with a nice crack and the cake is set when press gently on top, about 60 – 70 minutes. Since the cake is very moist, it’s ok to overbake. Transfer the pan to a wire rack and cool for 10 minutes. Loosen the sides with a butter knife or offset spatula. Carefully invert the pound cake onto a towel on top of the wire rack. Remove the pan, flip the cake back and place on the rack to cool completely. Slice and serve.