1. Whisk together cornstarch and water, then add to saucepan with raspberry jam. Stir until incorporated.
2. Over medium-low heat, bring the jam to a bare boil. When it begins to bubble, drop the heat to low and allow it to simmer for 2-3 minutes, until the cornstarch has begun to hydrolyze and thicken the jam. Remove from heat and cool before using as filling.
3. Heat your oven to 375˚
4. Cut six hearts out of each sheet of puff pastry, these will be the bottoms and tops of the pop-hearts. Peel away excess pastry and save for another us
5. Cover sheet tray with parchment paper and carefully transfer bottom hearts to the tray, leaving ~2 inches in between each heart for expansion.
6. Whisk together egg and milk for egg wash. Brush bottom hearts with egg wash.
7. Place two tablespoons of the cooled jam filling in the center of each bottom heart.
8. Carefully place the top heart over the jam filling and crimp edges all around to seal the dough.
9. Brush top of dough with egg wash.
10. Bake hearts on parchment lined sheet for 20-25 minutes, rotating the tray halfway through to ensure even color. The hearts are done when the dough is golden brown and puffed up.
11. Remove hearts from oven and allow to cool on tray for 5 minutes before using a spatula to remove them to a cooling rack.
12. While the hearts are cooling, make the glaze.
13. Whisk together powdered sugar, raspberry jam, and milk until thin enough to drizzle.
14. Drizzle raspberry glaze over cooling hearts, allow to set for a few minutes before serving.