Place the butter and sugar in an electric stand mixer with the flat paddle attachment.

Cream the butter and sugar until it is light and fluffy.

Add the egg and vanilla.

Slowly add the dry ingredients.

Keep beating until all of the ingredients are fully incorporated and you have a soft and smooth dough.

Transfer the dough to a large sheet of plastic wrap.

Roll up the dough in the plastic and tighten the ends to produce a fat log of dough about 8 cm in diameter.

Place the dough to chill in the fridge for 1-2 hours, or until it is completely firm.

Whisk the ingredients together in a small saucepan.

Place the saucepan over low heat.

Keep whisking until the ingredients form a thick paste. The consistency should be thick enough so that as you whisk the mixture, the whisk leaves lines in the mixture which remains.

Remove the paste to a small bowl, and set it aside to cool down.

Measure the flour, yeast, salt and sugar into the bowl of an electric stand mixer.

Mix the dry ingredients together using a dough hook.

Add the egg and the cooled Tangzhong.

Slowly add the milk until the mixture comes together into a sticky dough. 

Add the butter, one tablespoon at a time. Wait for each tablespoon of butter to be fully incorporated into the dough before adding the next.

Continue kneading the dough on medium speed for about 10-15 minutes, or until the dough passes the windowpane test. (See Kitchen Notes below). By this stage, the dough should also be coming away from the sides of the bowl.

Lightly oil a large, clean mixing bowl.

Roll the dough into a smooth ball and place it into the mixing bowl.

Cover the dough with a clean tea towel or bowl cover, and place it somewhere warm for about 1.5 hours, or until it has doubled in size.

Gently remove the dough from the bowl. The dough will deflate as you handle it.

Divide the dough into 12 equal portions. Use a digital scale for even-sized buns.

Roll each piece of dough into a smooth ball. I do this by pulling the edges of the dough into the centre, and then pinching those edges together while shaping the dough into a smooth ball as you do so. The pinched edges form the bottom of the buns.

Line 2 large baking trays with baking paper.

Place the shaped buns into the lined baking pan, spaced well apart.

Place the pan somewhere warm, covered with a clean tea towel, for about 30 minutes, or until the buns have almost doubled in size.

Remove the cookie dough from the fridge.

Cut the dough into 12 equal slices, about 0.5 cm thick.

Place the cookie slices onto a baking tray lined with baking paper.

You can leave the cookie slices as they are, or you can use the back of a knife or similar to create a crosshatch pattern on each cookie slice. The cookie will crack during baking, but creating a crosshatch pattern in the dough will help to create the pineapple look.

If the cookie slices feel very soft, you can place the tray back in the fridge or freezer to firm up again.

Once the buns have finished proofing, carefully place a cookie slice on top of each bun. Take care not to deflate the buns as you do so.

Brush the buns all over, including the cookie slices, with the egg wash.

Place a metal baking tray on the middle shelf.

Bake the buns for about 20 minutes, or until they are lightly golden. Check the buns at about 10-12 minutes and, if they are browning too quickly, cover the buns with a loose sheet of foil for the remaining baking time.

The buns are cooked if an internal thermometer reads 85°C/185°F.

Leave the buns on the pan for about 5 minutes to firm up, before carefully removing the buns to a wire rack to cool completely.

Serve warm or at room temperature.