Preheat the oven to 350°F.
Lightly butter a 9 inch round cake pan, line the bottom with a round of parchment paper, butter the parchment paper, and then flour the pan.
Sift the cake flour, baking powder, baking soda, and salt into a small bowl.
2 cups (256 grams) cake flour, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt
Using a stand mixer or a hand mixer and medium bowl, cream the butter, sugar, Earl Grey tea, and lavender until the butter is light and fluffy, scraping down as necessary, about 3 to 5 minutes.
1 stick (113 grams or 8 tablespoons) unsalted butter, softened, 1 cup (200 grams) granulated sugar, 1 tablespoon loose Earl Grey tea (or 2 tablespoons from 4 tea bags), ½ teaspoon ground culinary lavender
Add the eggs and vanilla, one addition at a time, beating on medium speed for a minute after each addition and scraping the bowl down as necessary.
2 eggs, 1 teaspoon vanilla
Add approximately half of the flour mixture, and beat on low until it is just incorporated. Add the buttermilk, and beat until just incorporated. Add the remaining flour and beat until just incorporated. During this process, scrape down the bowl as necessary.
1 cup buttermilk
Pour the cake batter into the prepared pan, and give the pan a few thwacks on the countertop to remove any air bubbles.
Bake on a middle shelf in the oven for about 40 minutes, until a toothpick into the center comes out free of crumbs. I usually start looking at cake layers 5 to 10 minutes before and start checking with a toothpick very few minutes once the cake stops wobbling.
Cool for 10 minutes in the pan, and then turn the cake out onto a cooling rack and let cool completely before frosting.