Take the puff pastry out of the freezer. Thaw at room temperature until softened.
Peel the kabocha squash and cut into 1-cm cubes. Place it in a Lekue steamer case and add some water.
Cover the case with the lid and microwave it at 600 W for about 3 minutes. Let it sit for 2 minutes to allow it to steam.
Add the butter to the kabocha while it is still hot. Use a potato masher to mash the kabocha squash. Add the sugar and milk and mix well.
Dust your working surface with cake flour. Roll out the puff pastry with rolling pin until it doubles in size.
Cut the puff pastry into 4 equal sections. Place 1/4 of the filling from Step 4 onto each section. Brush on the beaten egg yolk onto the bottom half of the puff pastry. (Reserve a small amount of egg yolk.)
Fold the top half of puff pastry onto the bottom half. Using a fork, press to seal the 3 edges.
Pierce some holes in the top puff pastry from Step 7 with a fork.
Place the dough from Step 8 onto a parchment-paper-lined cookie sheet. Brush the reserved egg yolk on top for glazing.
Bake the dough from Step 9 in a preheated oven at 200℃ for about 12 minutes or until the pies puff up and the tops become golden.
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Story Behind this Recipe
Since I was able to cook the kabocha squash easily using a Lekue silicone steamer, I made it into a pie.
Refer to the instructions on the packaging of the puff pastry for the time required to softened the pastry and the baking time. If the pies brown unevenly while cooking, rotate the cookie sheet.