Step 1: Prepare the Chocolate Layer

1. Melt the Chocolate: In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer. Remove from heat and stir in the dark chocolate until completely melted and smooth. Add the sugar and vanilla extract, stirring to combine.
2. Cool: Let the chocolate mixture cool slightly, but not completely—just enough so it won’t melt the semifreddo base when you fold it in.

Step 2: Prepare the Raspberry Layer

1. Blend the Raspberries: In a blender or food processor, combine the raspberries, sugar, lemon juice, and vanilla extract. Puree until smooth.
2. Strain (Optional): For a smooth raspberry layer, you can strain the mixture to remove the seeds. This is optional, depending on your texture preference.

Step 3: Make the Semifreddo Base

1. Whip the Cream: In a large mixing bowl, beat the chilled heavy cream until stiff peaks form. This will give the semifreddo its light and fluffy texture.
2. Fold in the Condensed Milk: Gently fold in the sweetened condensed milk and vanilla extract into the whipped cream, being careful not to deflate the whipped cream.

Step 4: Layer the Semifreddo

1. Layering the Mixtures: In a loaf pan or any freezer-safe dish, spoon half of the semifreddo base into the bottom. Smooth it out into an even layer.
2. Add the Chocolate Layer: Drizzle half of the melted chocolate mixture over the base, and gently swirl with a knife to create a marbled effect.
3. Add the Raspberry Layer: Spoon half of the raspberry mixture on top and gently swirl again.
4. Repeat: Repeat the process with the remaining base, chocolate, and raspberry mixtures until all the ingredients are used up.
5. Freeze: Cover the dish with plastic wrap and freeze for at least 4-6 hours, or overnight if possible, to allow the semifreddo to firm up.

Step 5: Serve and Enjoy

1. Slice: Once the semifreddo is frozen, remove it from the freezer and let it sit at room temperature for 5-10 minutes to soften slightly. Slice into portions, and serve immediately.
2. Garnish: If desired, top with fresh raspberries, a drizzle of melted chocolate, or a dusting of cocoa powder.