Step 1

Preheat oven to 425 degrees. Using a 2 1/2-inch heart cookie cutter and a pencil, trace 12 hearts on a piece of parchment paper, spacing 1 inch apart. Repeat with another piece of parchment. Lightly coat 2 rimmed baking sheets with cooking spray; line with prepared parchment, traced side down. Set aside.

Step 2

Make the pate a choux: Sift flour and cocoa powder into a medium bowl. Put milk, butter, sugar, and salt into a large saucepan. Bring to a boil over medium heat, stirring until sugar has dissolved. Remove from heat, and add flour mixture all at once. Stir vigorously with a wooden spoon until flour has been incorporated.

Step 3

Set pan over medium-low heat. Continue to cook, stirring, until dough pulls away from sides of pan and forms a ball, about 1 minute. Transfer dough to the bowl of an electric mixer fitted with the paddle attachment. Mix on medium-low speed until just warm to the touch.

Step 4

Raise speed to medium. Add eggs, one at a time, mixing well after each addition. Transfer dough to a pastry bag fitted with a 1/4-inch plain tip (such as Ateco #802). Pipe onto prepared parchment, tracing outlines of hearts and then filling centers. Pipe another heart on top of first layer.

Step 5

Place hearts in oven. Reduce temperature to 400 degrees. Bake 10 minutes, then rotate sheets. Reduce to 350 degrees; bake 7 minutes. Transfer baking sheets to wire racks to cool slightly. Turn off oven.

Step 6

Using a serrated knife, halve hearts horizontally. Arrange, cut sides down, on wire rack. Place rack on baking sheet; transfer to oven. Prop open door slightly, and let hearts cool completely, 45 to 60 minutes. Remove from oven; set aside.

Step 7

Make the filling: Prepare an ice-water bath; set aside. Pulse chocolate in a food processor until finely chopped. Put 1 cup cream and the sugar into a medium saucepan. Bring to a simmer, stirring until sugar has dissolved. Add hot cream mixture to the food processor. Carefully pulse until completely smooth. Transfer to a large bowl set in the ice-water bath. Let cool completely, whisking occasionally, 30 to 45 minutes. Pour through a fine sieve into a medium bowl.

Step 8

Put chocolate cream into the bowl of an electric mixer fitted with the whisk attachment. Add remaining cup cream. Beat on low speed until medium peaks form (do not overbeat).

Step 9

Make the glaze: Pulse chocolate in a food processor until finely chopped. Put cream and corn syrup into a medium saucepan. Bring to a simmer, stirring until syrup has dissolved. Add hot cream mixture to the food processor. Carefully pulse until completely smooth. Pour through a fine sieve into a bowl; use a rubber spatula to eliminate air bubbles.

Step 10

To assemble: Spoon glaze over tops of hearts. Let stand until set, about 10 minutes. Meanwhile, spread filling onto cut sides of bottoms of hearts. Sandwich tops and bottoms. Serve immediately, or refrigerate, uncovered, up to 4 hours.