01 - In a small saucepan, combine the raspberries, granulated sugar, and lemon juice. Cook over medium heat, stirring frequently, until the berries soften and the mixture thickens, approximately 5 to 7 minutes. Press the mixture through a fine-mesh sieve to remove any seeds, then set aside to cool completely.
02 - Place the chopped semi-sweet chocolate in a heatproof bowl. Melt the chocolate either over a saucepan of simmering water (creating a double boiler) or in a microwave using short intervals, stirring until smooth. Allow the melted chocolate to cool slightly.
03 - In a clean, dry mixing bowl, whip the egg whites with a pinch of salt until soft peaks begin to form. Gradually incorporate the granulated sugar, continuing to beat until stiff, glossy peaks are achieved.
04 - In a separate chilled bowl, whip the heavy cream until soft peaks form. Gently fold in the pure vanilla extract.
05 - Whisk the egg yolks into the lukewarm melted chocolate until the mixture is thoroughly combined and smooth.
06 - Gently fold the whipped cream into the chocolate and egg yolk mixture until just combined, being careful not to deflate the cream.
07 - Delicately fold the beaten egg whites into the chocolate mixture in two additions, using a light hand to preserve the airy texture of the mousse.
08 - Spoon an initial layer of the prepared raspberry purée into the bottom of each individual serving cup or glass. Carefully top with a layer of the chocolate mousse. Repeat the layering process if desired, finishing with mousse.
09 - Refrigerate the assembled mousse cups for a minimum of 2 hours, or until the mousse is fully set and firm.
10 - Prior to serving, embellish each mousse cup with fresh raspberries, dark chocolate shavings or curls, and an optional sprig of mint.