Preheat oven to 350°F. Grease a 9-inch cake pan with butter or cooking spray and set aside. 

Steep tea in ½ cup of boiling water for 5 minutes. Add milk and set aside. 

In a large bowl, add sugar and lemon zest. Use your hands to rub the zest into the sugar for 1 minute. This helps express all the oil from the zest, giving your cake a more lemony flavor. Add the butter, baking powder, and salt. Mix with a hand mixer for 2 minutes. The mixture will look crumbly. Add the egg and vanilla extract, and mix for 4 minutes. The batter should look light and fluffy. Alternately add in the Earl Grey tea mixture and sift in the flour in three additions, mixing lightly between each addition. Once the batter is well combined, pour the batter into the prepared pan. 

Bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool for 10 minutes before turning out onto a rack to cool completely. 

When the cake is cool, make the whipped cream. Add heavy cream, powdered sugar, and vanilla to a mixing bowl and whisk until you have stiff peaks; this should take about 4-5 minutes. Spoon the whipped cream onto the cooled cake and use a spoon to spread it evenly. Dust with some Earl Grey tea and garnish with lemon zest before serving.