Place sifted matcha and cornstarch in a small to medium heat-safe bowl.
In a saucepan over medium heat, pour milk, sugar, vanilla, and salt.
Once warm to the touch (don't let it simmer or boil) and the sugar is dissolved, add a ladle-full or a few tablespoons of the warm liquid to the sifted matcha and cornstarch. Whisk until completely smooth, then pour it all back into the pot.
Continue whisking over medium heat until the mixture is thickened and coats the back of a spoon.
Pour the cream over a fine mesh sieve into a medium heat-safe bowl to get rid of any lumps. Push the cream through the sieve by using a whisk or rubber spatula.
Plastic wrap the cream and place it in the fridge. Once fully cooled, transfer to a piping bag. While waiting for it to cool, you can start the matcha dough.
For the matcha dough:
In a large bowl or bowl of a stand mixer fitted with a dough hook, whisk the yeast and sugar in warmed milk until fully dissolved.
Add the oil, bread flour, salt, and sifted matcha, and knead until smooth, about 5-8 minutes. The dough should be soft, workable, but slightly sticky.
Cover the dough and let it rise in a warm place until doubled in size, about one hour.
Forming the frogs:
Set up a baking tray with parchment paper or a silicone mat. Lightly dust your workspace with flour.
Punch the dough down, set it on your workspace, and split into six 45g pieces. Keep the leftover dough—this will be for the eyes.
Shape the seven pieces into balls by cupping it with your palm and moving your hand in circular motions over the workspace. Cover the dough balls with plastic wrap and let it sit for 10 minutes. This lets the gluten relax, which will make shaping much easier for the next step.
Using a rolling pin, flatten one ball of dough into a circle. Pipe a large dollop of matcha cream in the middle, then pinch the edges of the dough together to secure the cream inside. Gently shape into a ball, then place on a lined baking tray. Repeat with the other five 45g balls, spacing them about 3-4 inches apart on the tray.
Take the leftover dough and split it into 12 equal balls for the eyes. Attach the eyes to your frogs using water as glue.
Lightly dust your frogs with flour. Loosely cover them with plastic wrap and let rise until doubled in size, about one hour. During the last 5 minutes of rising, preheat your oven to 300°F/150°C.
Bake and decorate:
Bake the frogs for about 15 minutes, or until edges are slightly firm. The buns will not turn golden brown due to the green color from the matcha.
Let cool on tray for about 5 minutes, then transfer to wire rack.
Decorate the faces using black and pink edible food pens. Alternatively, you can use melted chocolate.
Enjoy!
Notes
The buns are best eaten fresh, but you can also store them in an airtight container in the fridge for up to 3 days. If you want to reheat them, place the bun on a microwave-safe plate covered with a damp paper towel, and heat for 10 seconds.