In a medium saucepan, whisk together the sugar, flour, and instant tea powder, then whisk in the milk and the cream.
Heat the mixture on medium-high heat, whisking fairly often, especially when the mixture comes to a boil. I heated this mixture for a total of 11 minutes until it was super thick and gooey.
Strain through a sieve into the bowl of a mixer, working it through with your whisk, then beat the mixture to cool (this takes a long time) with the paddle attachment.
When you are sure the sweet tea mixture is completely cooled, add in the butter in dollops, while mixing on low, then when all the butter is added, increase the speed to medium-high and whip the frosting until it is light and fluffy.
Level the cakes with a serrated knife, then place the first layer on a cake stand. Top with a couple spoonfuls of frosting (~70 grams) and smooth it out. Stack on the second cake layer and another 70 grams of frosting, then top with the final layer and frost the sides and top with the last of the frosting.
Decorate with a ring of loose-leaf Earl Grey tea before serving.