Prepare the Peach Filling: In a saucepan over medium heat, combine cubed peaches, granulated sugar, and lemon juice. Cook for 12–15 minutes, mashing occasionally until the mixture is juicy and thickened. In a small bowl, stir together cornstarch and water, then mix it into the peach jam. Cook for an additional 2 minutes, allowing it to thicken further. Transfer the thickened peach filling to a bowl and let it cool completely.
Create the Tangzhong: In a small saucepan, whisk together water and flour over medium heat. Cook this mixture while constantly whisking for 4–5 minutes, or until it becomes thick and paste-like. Once thick, transfer it to a bowl and allow it to cool.
Make the Dough: In the bowl of a stand mixer, combine all-purpose flour, granulated sugar, instant yeast, and fine sea salt. Add in the warmed milk, large egg, vanilla extract, and the cooled tangzhong. Mix on low speed until the dough forms a shaggy consistency. Gradually introduce the softened butter, one tablespoon at a time, mixing until fully incorporated. Knead the dough on medium speed for about 10–11 minutes until it is smooth and elastic. Form it into a ball and place it in a greased bowl, covering it to rise in a warm place for about 1 hour, until doubled in size.
Prepare the Crumb Topping: In a mixing bowl, combine melted butter with all-purpose flour, brown sugar, and ground cinnamon. Mix until the mixture is crumbly, resembling coarse sand. Once mixed, chill the crumb topping in the refrigerator until you are ready to use it.
Roll Out the Dough: After the dough has risen, turn it out onto a floured surface and roll it into an 18×15-inch rectangle. Spread the brown sugar filling evenly over the dough, then spoon the cooled peach jam on top. Layer some thinly sliced peaches on top of the jam for extra flavor. Cut the dough into 12 strips, each about 1½ inches wide, and roll each strip tightly into swirls.
Second Rise: Carefully place the rolled swirls into a parchment-lined 9×13-inch metal baking dish. Cover them with a towel and let them rise again for roughly 1 hour, or until they are doubled in size.
Bake the Rolls: Preheat your oven to 325°F (162°C). Once the rolls are puffed up, generously sprinkle the chilled crumb topping over the top. Bake them in the preheated oven for about 30–35 minutes, or until they are golden brown on top and look deliciously risen. Once baked, allow them to cool slightly in the pan.
Make the Frosting: In a mixing bowl, combine cream cheese with softened butter using an electric mixer until the mixture is smooth and creamy. Gradually add powdered sugar while mixing, and drizzle in heavy cream to achieve your desired frosting consistency.
Frost the Rolls: Once the rolls have cooled slightly, generously drizzle the cream cheese frosting over the warm rolls, allowing it to melt deliciously into every crevice.