Cut 6 ounces of semisweet chocolate into small, evenly sized pieces and transfer them to a dry heatproof bowl.
Pour ½ cup heavy cream into a small saucepan. Warm over medium-low heat for 3 to 5 minutes, until steaming with tiny bubbles around the edge. Do not boil.
Pour the hot cream over the chopped chocolate. Leave it untouched for 2 minutes, then stir slowly until completely smooth and glossy.
Leave the mixture at room temperature for 8 to 10 minutes. It should remain warm but should no longer feel hot.
Pour the remaining 1 cup cold heavy cream into a clean bowl. Add 1 teaspoon vanilla extract and beat for about 2 to 3 minutes, until soft peaks form.
Fold about one-third of the whipped cream into the chocolate to loosen it. Fold in the remaining cream with broad, gentle strokes until evenly combined and fluffy.
Reserve about ¼ cup raspberries for topping. Mash the remaining 1¼ cups with 2 tablespoons sugar until juicy but still slightly chunky. Rest for 5 minutes.
Divide the raspberry mixture among 6 dessert cups, using about 2 tablespoons per cup.
Pipe or spoon the chocolate mousse over the raspberry layer until each cup is about ¾ full.
Cover and refrigerate for at least 2 hours, or 3 to 4 hours for a firmer set.
Top each cup with 2 to 3 fresh raspberries and a small pinch of flaky sea salt right before serving.