Into the bowl of a stand mixer fitted with a dough hook, add the bread flour, sugar, salt, instant yeast, vanilla extract, egg, and milk. Knead for about 3 minutes, or until a sticky dough forms.

Add the butter and continue kneading for 12–15 minutes, or until the dough passes the windowpane test - you should be able to stretch a small piece into a thin, translucent sheet without tearing. The dough should be smooth and elastic.

Cover the bowl with plastic wrap and let it proof at room temperature for 1–1.5 hours, or until doubled in volume.

Meanwhile, prepare the cookie topping. In a medium bowl, beat the butter and sugar until light and fluffy.

Add the egg and matcha powder, and beat until the mixture is smooth and well combined.

Fold in the flour using a rubber spatula until no dry flour remains.

Divide the cookie dough into 6 equal portions. Roll each portion into a ball.

Place each ball between two pieces of parchment paper and roll it into a disc about 4½ inches (12 cm) in diameter. Peel off the top layer of parchment paper, replace it with a piece of plastic wrap, then flip the whole thing over and remove the remaining parchment paper. (You can absolutely roll the dough between two pieces of plastic wrap and simply peel one off, leaving the other attached to the dough, but I prefer using parchment paper for rolling.) You should now have a cookie dough disc resting on a piece of plastic wrap. Repeat with the remaining cookie dough.

Stack the discs on top of each other so the plastic wrap separates them to prevent sticking. Refrigerate until ready to use.

Once the bread dough has finished proofing, punch it down to deflate and transfer it to a lightly floured surface. Divide it into 6 equal parts and roll each into a smooth ball.

Take one cookie dough disc, using the plastic wrap to help handle it, and place a dough ball in the center. Gently wrap the cookie disc over the top, sides, and just slightly onto the bottom of the bread dough. Do not fully encase the dough in the cookie topping. Leaving the bottom slightly exposed allows the cookie layer to expand properly during baking. If needed, trim any excess cookie dough from the bottom using a small knife.

Carefully press the cookie dough to smooth any creases. Then, flip the assembled bun and lightly press the top into a plate of granulated sugar, coating the surface evenly.

Flip the bun back over and use the back of a knife or a dough scraper to gently score a criss-cross pattern on the sugared surface to resemble a melon’s rind. Be careful not to cut through the cookie topping. Repeat with the remaining dough balls and cookie discs.

Place the assembled buns on a parchment-lined baking sheet, spaced at least 2 inches (5 cm) apart. Cover loosely with plastic wrap and let them proof for another 40–60 minutes, or until slightly puffy.

Preheat your oven to 190C (375F).

Once the buns have finished proofing, bake for 15 minutes, or until the buns are expanded and the cookie crust is lightly golden.

Transfer to a wire rack to cool completely.