Pour 45ml (3 tbsp) of cool water into a small bowl. At a height, sprinkle the gelatin over the water, ensuring that it covers the surface of the water evenly. Set aside and allow the gelatin to hydrate for at least 5 minutes.
Meanwhile, add the raspberries and 20g (2 tbsp) of sugar to a small saucepan. Set over medium heat and, stirring frequently, bring to a simmer. Allow the raspberries to simmer and break down for about 5 minutes. Set a fine mesh sieve over a small bowl and strain the raspberries through the sieve, pressing to ensure everything passes through and only the seeds remain in the sieve.
Add the hydrated gelatin (it will likely come out as one large puck) to the pan in which you cooked the raspberries - there is no need to clean it out - and allow the residual heat to melt the gelatin. Once melted, stir it into the raspberry mixture and set aside.
Meanwhile, add a few centimetres (about 2 inches) of water to a small saucepan and set over medium-low heat. Bring to a simmer. Add the chocolate and cream to a large heatproof bowl and set over the simmering water, ensuring the water is not touching the bowl. Gently melt the chocolate and remove from the heat.
Once the chocolate is melted, add the egg yolks and whisk vigorously until thoroughly combined and the mixture is glossy and lightened. Scrape in the raspberry and gelatin mixture and stir until combined.
Add the egg whites, cream of tartar and a pinch of salt to a large bowl or the bowl of a stand mixer fitted with the whisk attachment. Using a hand mixer (or stand mixer) whip the whites on medium speed until they're foamy - about 1 minute. Increase the speed and with the mixer running, slowly add the remaining 30g of sugar. Continue whipping until the sugar is dissolved in the meringue and it holds stiff peaks.
Using a large flexible spatula, transfer about ⅓ of the meringue to the chocolate mixture and fold until incorporated. Add the rest of the meringue in two batches, folding gently so as not to deflate the meringue.
Transfer the mousse to serving dishes or glasses and move to the refrigerator. Allow to chill for at least 3 hours and, covered, up to 3 days before serving.
Notes
If using frozen raspberries, there is no need to defrost.
I prefer caster (aka superfine) sugar over granulated sugar as it dissolves more easily into the meringue. That being said, granulated sugar can be just as easily used, just note it may not dissolve into the meringue as quickly.
The meringue can be made without cream of tartar; it simply helps to stabilise it. You can substitute with ½ tsp of white vinegar or leave it out entirely.