Failproof flakey pastry stuffed with mochi and chocolate

Ingredients

FAILPROOF FLAKEY PASTRY:
• 1 1/2 cup (195 grams) all-purpose flour
• 1/2 tsp sea salt
• 1/3 cup + 2 tbsp (115 grams) warm water at 130-140F/55-60 C
• 4 1/2 tbsp (63 grams) unsalted butter, cold
• 1 egg, beaten
MOCHI:
• 3/4 cup (85 grams) sticky rice flour
• 2 tbsp (25 grams) light brown sugar
• 1/3 cup + 1 tbsp (95 grams) whole milk
CHOCOLATE FILLING:
• 3.5 oz (100 grams) bittersweet chocolate
• 1/3 cup (65 grams) dark brown sugar
• 1/8 tsp ground cinnamon
• 2 tbsp (32 grams) peanut butter

Instructions

1. PREPARE PASTRY AND FILLING: Microwave 1 1/2 cup of water on high for about 1:30 min to bring it to a temperature of 130-140F/55-60 C. I recommend measuring it with a thermometer to be sure. Inside a large bowl, add flour, sea salt and 1/3 cup + 2 tbsp (115 grams) warm water, and mix until a craggy but even and cohesive dough forms. There no need to knead it further because we don't want much gluten to develop (in fact the warm water is to eliminate gluten). Cover the bowl and flash-freeze in the freezer for 15 min until the dough is cool to the touch.
2. Meanwhile, whisk together sticky rice flour, light brown sugar and whole milk in a microwave-safe bowl until even. Microwave the mixture on medium power for 45 seconds interval for about 4 to 5 times, stirring with a fork in between, until the mixture comes together as a sticky and stretchy dough. Keep stirring and kneading if you will with a fork until the dough is very even and bouncy. Cover with plastic wrap and chill in the fridge until needed.
3. In a food-processor, pulse bittersweet chocolate, dark brown sugar and ground cinnamon until coarsely ground. Add peanut butter and pulse until the mixture resembles wet sand. Keep in the fridge until needed.
4. SHAPE THE PASTRY: Transfer the flour dough onto a well-floured working surface and roll it out into a rectangle (keep it as neatly rectangular as you can) about 8"x11", the size of A4 paper. Use a pastry cutter to cut the butter into thin plates and evenly arrange them to cover 2/3 of the rectangular dough but leaving about a 3/4" (2cm) margin all around. If the butter is not covering the area very evenly, you can spread it out a little with a butter knife, no worries.
5. Brush the egg all around the margin, then fold the unbuttered 1/3 of the dough over the buttered area. Pat on it with some pressure to eliminate major air pockets, then fold the bottom 1/3 of the buttered dough over again, pat on it, then pinch on the margin to make sure the edges are sealed. Cut off some excess edges if any.
6. Then rotate the dough 90 degrees, roll it out into the size of an A4 paper again. Some butter may squeeze out from the edges but listen, IT'S NOT A BIG DEAL. Simply smear it back onto the dough wherever, no big deal!! If there are large air bubbles being pushed around, poke it with a wooden skewer. If the dough or squeeze out butter is making it stick or messy, dust over with more flour. REPEAT AFTER ME, IT'S NOT A BIG DEAL. Fold the rectangular dough the same way again, 1/3 towards to the middle and the other 1/3 over again. Rotate 90 degrees, and repeat one more time. So basically 3 foldings. Now flash-freeze the folded dough in the freezer for 20 min. Meanwhile, dust the chilled mochi dough with cornstarch to prevent sticking, and roll it out into a rectangle about 5"x8" (13x20 cm), set aside until needed.
7. Now place the chilled dough on the working surface seam-side down, and roll it out again into a larger rectangle about 10"x15" (26x38 cm), thinner than a typical pie crust. Use a pastry cutter to cut off the curving edges to make it a perfect rectangle. Spread the chocolate-mixture in the middle covering an area little more than 1/3 of the width with a 3/4" margin (see photo for guidance). Place the mochi sheet on top, which should fit perfectly. Brush the beaten egg around the margin, then fold each side of the dough towards the middle to close (you should have a partial overlap of dough). Pinch the edges to seal tightly, then flip the whole thing over.
8. You can bake it immediately, or chill in the fridge for later. Either way, brush the whole surface with beaten egg and bake it in a 400 F/200 C oven with FAN-ON (or 430F/220C without fan) for 35 min, rotate half-way, until golden-browned all over. Let cool on a rack for 20 minute before serving. It will be much easier to cut with a scissor than a knife.