For the craquelin (cookie crust), in a food processor, pulse the butter, light brown sugar and salt until just blended. Add flour and pulse until the mixture looks moist and crumbly. Add the vanilla and pulse until combined. Transfer the dough to a clean surface and form into round disk. Place the round disk in between 2 parchment papers. Roll the dough out until it is 1/8 – 1/16-inch thick. Transfer the dough along with the parchment papers onto a baking sheet and freeze for 30 minutes. Cut the dough into 1½-inch rounds with a cookie cutter. Freeze the craquelin rounds for at least 1 hour or until ready to use. Discard the rest of the dough.
For the milk tea pastry cream, in a small saucepan over medium heat, bring milk to a just a simmer. Remove from heat. Add tea leaves and tea bag. Give a quick stir. Cover with a lid and steep for 30 minutes.
In a medium mixing bowl, stir together sugar, cornstarch and salt. Whisk in egg yolks until mixture is pale yellow and fluffy, about 2 minutes. The mixture will be pretty thick.
Strain the tea mixture into a measuring cup. Discard the tea leaves and tea bag. Wipe the saucepan clean with a paper towel. While whisking the yolk mixture, slowly pour in milk tea. Return the mixture back to the saucepan. Cook over medium heat, whisking constantly until thicken, about 3 minutes. Pause to check for bubbles. When it starts bubbling, whisk for 30 more seconds. Remove from heat. Whisk in butter until combined. Transfer to a heatproof medium bowl. Cover with plastic wrap by pressing down to the surface of the pastry cream. Chill in the fridge for 3 hours or until cold.
For the coffee pastry cream, in a small saucepan over medium heat, bring milk to a just a simmer. Remove from heat. Add instant coffee. Give a quick stir. Cover with a lid and steep for 30 minutes.
In a medium mixing bowl, stir together sugar, cornstarch and salt. Whisk in egg yolks until mixture is pale yellow and fluffy, about 2 minutes. The mixture will be pretty thick.
While whisking the yolk mixture, slowly pour in coffee milk. Return the mixture back to the saucepan. Cook over medium heat, whisking constantly until thicken, about 3 minutes. Pause to check for bubbles. When it starts bubbling, whisk for 30 more seconds. Remove from heat. Whisk in butter until combined. Transfer to a heatproof medium bowl. Cover with plastic wrap by pressing down to the surface of the pastry cream. Chill in the fridge for 3 hours or until cold.
For the cream puffs, preheat oven to 375˚F (190˚C). Line 2 baking sheets with parchment papers. Set aside.
In a small bowl, lightly beat together eggs and egg white. Set aside.
In a small saucepan over medium-high heat, bring the milk, water, butter, sugar and salt just to a boil, stirring occasionally. Reduce the heat to low. Add the flour all at once and stir until dough forms into a smooth ball and pull away from sides of the pan. Keep stirring until thin-film forms on the bottom of the pan, about 2 – 3 minutes.
Transfer the hot dough to a large mixing bowl. With an electric mixer fitted with a paddle attachment, mixing on medium-speed for 1 minute to release hot steam. Add 1/3 of the lightly beaten eggs, mix on medium-speed until blended. Add half of the eggs and mix again. Add 1/3 of the remaining eggs, and beat. Check the dough consistency. The dough should be smooth with a glossy sheen, but also stiff enough to be pipeable. You may not need to add all the eggs.
Transfer the dough to a pastry bag fitted with a ½-inch round tip. Hold the pastry bag straight up and pipe out 12 (1½-inch) mounds on a prepared baking sheet, spacing about 2 inches apart. Do not pipe the other baking sheet yet (bake 1 sheet at a time). Carefully top each dough mound with a frozen craquelin round and press gently to stick.
Bake in the oven for 10 minutes. Reduce heat to 350˚F (177˚C). Continue to bake for 20 – 25 minutes, until the tops are golden brown, the puffs feel firm when squeezed gently and sound hollow when tapped on the bottom. Do not open oven door during baking. Remove from oven and transfer cream puffs to a cooling rack. While they are still hot, carefully poke a small hole at the bottom center of the cream puff with a paring knife to release any steam inside. Cool completely. Repeat with the second baking sheet. Remember to turn up the oven to 375˚F (190˚C) to start.
For the stabilized whipped cream, in a small microwave-proof bowl, add water. Sprinkle the gelatin powder over evenly. Set aside to soften for 10 minutes. Microwave for 10 – 15 seconds. Stir to dissolve.
In a large mixing bowl with an electric mixer fitted with a whisk attachment, add heavy cream and powdered sugar. Whisk for 1 minute at medium-high speed. Pour the warm gelatin liquid into the cream. Continue to whisk until soft peaks form. Cover and keep in the fridge until ready to use.
When almost ready to serve, prepare the cream fillings. In two medium mixing bowls, add milk tea pastry cream in one bowl and coffee pastry cream in another. Whisk them separately to smooth out. Add 1/3 of whipped cream into each pastry cream. Gently fold in the whipped cream until blended.
There are 2 ways to fill the cream fillings into pastry bag(s). Make sure the pastry bag(s) is fitted with a round or star tip. Method 1: Fill both milk tea and coffee cream filling side by side into a single pastry bag. Use a small spoon to place the cream into the bag. It won’t be perfect, but it’s ok. Method 2: Fill the 2 cream fillings into 2 different pastry bags. One flavor in one bag.
To assemble, hold a cream puff on one hand, and hold a pastry bag on another hand. Insert little bit of the tip of the pastry bag into the small hole at the bottom of the puff. Squeeze the cream filling into the puffs. When you see the filling starts to spill out the sides of the hole, it’s full. Continue to fill the rest of the cream puffs. If using 2 pastry bags (one flavor in one bag), fill one flavor first, then fill the other. It will be a bit hard to tell how much you fill, but just do your best. It doesn’t have to be perfect. Serve immediately or within 30 minutes.