Make the Thai tea custard: Add the milk and Thai tea bags to a small saucepan. Warm over low heat until the milk just barely begins to simmer, about 5 minutes. Using a rubber spatula, gently press the tea bags against the side of the pot so that all of the flavor is released into the milk. Turn off the heat, remove the tea bags, and let the infused milk cool to room temperature, about 10 minutes.
2. Step
In a medium bowl, whisk together the egg yolks, sugar, sweetened condensed milk, and cornstarch until thoroughly combined and the mixture turns light yellow in color.
3. Step
While whisking constantly so the eggs do not curdle, slowly pour the infused milk into the egg yolk mixture. Pour the custard back into the pot used to infuse the milk and cook over medium heat, whisking constantly to prevent lumps from forming, for 3–4 minutes, until thickened. Strain the custard into a medium heatproof bowl and cover with plastic wrap, pressing directly against the surface to prevent a skin from forming. Transfer to the freezer to chill for 20 minutes, or refrigerate overnight.
4. Step
Make the wheel cakes: In a medium bowl, whip the egg whites with an electric hand mixer on medium speed until soft peaks form, 1–2 minutes.
5. Step
In a large bowl, whisk together the egg yolks and sugar until the mixture turns light yellow in color. Add the salt, milk, and melted butter and whisk to combine.
6. Step
Sift the flour and baking powder into the egg yolk mixture and whisk to combine.
7. Step
Using a rubber spatula, gently fold the egg whites into the batter a bit at a time, taking care not to deflate the batter.
8. Step
Heat the wheel cake pan over low heat. Generously grease the pan, as well as a ¼-cup measure and 1-tablespoon measure, with nonstick spray.
9. Step
Add a pinch of black sesame seeds to each of the wheel cake cups. Using the greased measuring cup, scoop ¼ cup of the batter into each of the cups. Using the back end of a chopstick, swirl the batter around so it coats the sides of the cups all the way to the top edges. Cook for 1–2 minutes, until the sides begin to bubble and set.
10. Step
Using the greased tablespoon measure, add 2 tablespoons of boba to the centers of two of the cakes. Add 2 tablespoons of Thai tea custard to each of the remaining cakes. Continue cooking the cakes, using chopsticks to rotate them so they cook evenly and don't stick to the pan, until browned and cooked ¾ of the way through, 2–3 minutes more.
11. Step
Use an offset spatula to flip the cakes with the boba onto the cakes with the tea custard, gently pressing down to sandwich the halves together. Continue to cook for 1–2 minutes, until the cakes are evenly brown on both sides. Remove the cakes from the pan and repeat with the remaining batter and fillings to make 7 cakes total.
