In a medium saucepan, whisk together the water, milk, and flour until the mixture is smooth and free of lumps. Place the saucepan over medium heat and, whisking constantly, cook for 2 to 3 minutes, until the mixture thickens to a smooth, pudding-like consistency. Remove from heat and set aside.

Add the melted butter and milk to the tangzhong in the saucepan, whisking until very smooth. Next, whisk in the eggs until fully incorporated. The liquid ingredients will cool the tangzhong, and the mixture should feel lukewarm at this point.

In the bowl of a stand mixer (or a large bowl if kneading by hand), combine the flour, powdered milk, and yeast by whisking them together. Pour in the tangzhong and milk mixture.

Fit the stand mixer with the dough hook and mix on low speed until the flour is moistened and a shaggy dough comes together, about 1 to 2 minutes. Cover the bowl and let the dough rest for 20 minutes to allow the flour to absorb the liquid, making it easier to knead.

After resting, add the sugar and salt, then mix on medium-low speed for about 10 minutes until the dough is smooth, elastic, and slightly sticky. It will be very sticky at first, but resist adding more flour— the dough will absorb the excess moisture as mixing continues. After kneading, the dough should feel slightly sticky but not messy, and it should form a smooth, taut ball when handled with lightly oiled hands.

Transfer the dough to a lightly greased large bowl and cover it tightly with plastic wrap. Allow it to rise in a warm place for about 1 hour, or until nearly doubled in size. For a slower rise, you can refrigerate it overnight—this method makes the dough easier to handle. Meanwhile, prepare the filling.

In a medium bowl, whisk together the brown sugar, cornstarch, cinnamon, and salt until well combined. Cover with plastic wrap and set aside until ready to use.

Once the dough has risen, gently deflate it and turn it out onto a lightly floured counter or pastry mat. Pat, stretch, and roll the dough into a 16x18-inch rectangle, with the long edge facing you.

Evenly spread the partially melted butter over the surface of the dough, then sprinkle the brown sugar filling over it, leaving a 1-inch border along the top edge. Use your hands to smooth the filling into an even layer and gently press it into the dough to help it adhere.

Starting from the long edge nearest you, roll the dough tightly into a cylinder. Pinch the seam to seal the dough shut and roll it seam-side down. Use a knife to mark the dough into 12 equal portions. For clean cuts, slide a strand of dental floss under the cylinder, cross the ends, and pull to slice. Cut the dough into 12 portions (about 2 inches each) and transfer them to a greased 15x13-inch rimmed baking sheet or a 13x9-inch baking pan.

Cover the pan loosely with plastic wrap and allow the rolls to rise until puffy and touching each other, about 40 to 50 minutes. In the last 15 minutes of rising, adjust the oven rack to the middle position and preheat the oven to 350°F (180°C). While the rolls rise, prepare the frosting.

In a medium bowl (or using a food processor), beat the butter and cream cheese together with an electric mixer for about 2 minutes, until smooth and creamy. Gradually add the powdered sugar while continuing to beat, until fully incorporated and the mixture is light and fluffy, about 2 more minutes.

Add the milk, lemon juice, vanilla, and salt, then beat until well combined. Cover and set aside.

Remove the plastic wrap from the rolls and bake them in the preheated oven for 20 to 25 minutes. Tent with aluminum foil halfway through baking to prevent over-browning. The rolls are done when the center of the dough registers around 190°F on an instant-read thermometer or when a toothpick inserted into the center comes out clean and smooth.

Once baked, remove the rolls from the oven and immediately spread the frosting on top while they’re still warm, letting it melt partially into the rolls. Serve warm.

Store any completely cooled rolls in an airtight container at room temperature for a few days.