Line a 9-by-13-by-2-inch baking pan with parchment paper. Set aside. Place raspberries (if using fresh raspberries, reserve 1/2 cup for garnish) in a medium skillet over medium heat. Simmer until raspberries begin to break down and liquid is released, 4 to 5 minutes. Transfer raspberries to a food processor, and puree, about 20 seconds. Pass through a fine sieve. Set aside to cool.
Fill a large bowl with ice and water; set aside. Bring a stockpot 1/4 full of water to a boil. Let simmer. In a medium stainless-steel bowl, whisk together egg yolks, 2 tablespoons sugar, corn syrup, and kirsch or champagne; place over simmering water. Whisking constantly, cook until mixture thickens (when whisk is lifted over the mixture, the ribbonlike trail it leaves on the surface should hold its shape for 1 or 2 seconds), about 4 minutes. Transfer bowl to the ice-water bath; whisk mixture while it cools. Add 3/4 cup raspberry puree to mixture; whisk to combine. Set aside.
In an electric mixer fitted with the whisk attachment, beat egg whites on high speed until frothy, about 30 seconds. While mixer is running, gradually add remaining 6 tablespoons sugar; beat until very stiff, 2 to 2 1/2 minutes. Transfer mixture to a bowl. Beat cream with whisk attachment until stiff; fold into egg whites.
Fold raspberry mixture into egg whites and cream. Pour the mixture into the prepared baking pan; cover with plastic wrap. Transfer to the freezer; freeze until hardened, at least 4 hours. This can be prepared up to 2 days in advance.
To make the raspberry sauce, stir superfine sugar into the remaining raspberry puree (about 2 tablespoons). Refrigerate until ready to serve.
To serve, use a heart-shaped cookie cutter (2 1/2 inches across at the widest point) to cut out four hearts from the semifreddo. Divide the raspberry sauce between two dessert plates. Place a chocolate heart, off center, on top of the raspberry sauce. Place a semifreddo heart on each chocolate heart, and repeat layering until there are three layers of chocolate and two layers of semifreddo, ending with a chocolate heart. Garnish with reserved fresh raspberries (if using), and sift confectioners’ sugar over the raspberries, if desired.