Center a rack in the oven and preheat to 350 (F). Prepare your cake pan by spraying generously with cooking spray. Seriously, if you're using a bundt mold, use a LOT. Don't forget to spray the metal dent in the middle of the mold. Set aside.
In the bowl of a food processor, combine 1 1/2 cups granulated sugar, the tea from 3 Pure Leaf Black Tea with Vanilla satchels, and fresh zest from 2 medium lemons. Pulse in short bursts for around 30 seconds, or until the lemon zest and tea has been ground into smaller pieces and is fully incorporated throughout the sugar. Don't be worried if your sugar clumps — this is normal and is just the zest releasing oil into the sugar to make it more flavorful.
In a medium bowl, whisk together 3 1/2 cups cake flour, 3 teaspoons baking powder, and 1/2 teaspoon kosher salt until well combined. Set aside.
In the bowl of a freestanding electric mixer fitted with a paddle attachment, combine the processed sugar (from the 2nd step) with 1 cup unsalted butter. Beat on medium speed until light and fluffy, at least 3 to 5 minutes. When the mixture is light and fluffy, lower the mixer speed to its lowest setting. Add 8 large egg whites, one egg white at a time, only adding the next egg white when the previous one has been incorporated. When all the egg whites have been incorporated, add 1 teaspoon pure vanilla extract.
Continue mixing the batter on low speed. Add the dry ingredients (from the 3rd step) in three parts, alternating 1 cup whole milk in 2 parts, starting and ending with the dry ingredients. Continue mixing until just combined.
Transfer to the prepared cake pan and bake in the preheated oven for 1 hour, or until the bottom of the cake is golden and springs back when gently touched. When the cake is ready, transfer to a wire rack and allow to cool in the pan. While the cake is cooling in the pan, make the lemon and black tea syrup.
In a small, heavy-bottomed saucepan over medium heat, combine 1/2 cup granulated sugar and 1/2 cup Pure Leaf Black Tea with Vanilla. Whisk until the sugar dissolves and bring the mixture to a boil. Boil for one minute, then transfer immediately to a liquid measuring cup and allow to cool slightly on a wire rack for 5 minutes. After the syrup has cooled slightly, use a wooden skewer or a fork to poke a bunch of holes onto the top of the still-inverted cake (so, technically, you would be poking holes on the bottom of the cake if it had been inverted out of its pan). Use a pastry brush to spread the warm syrup across the bottom of the cake, stroking the syrup around to encourage it to leak into the holes. Don't be shy — use up all the syrup here to get an incredibly moist and delicious cake!
Once you've completed "soaking" the cake with the lemon syrup, invert the cake onto its serving platter and allow to cool completely before glazing.