Heat the oven to 170°C/Fan 150°C/Gas 3.
Melt the chocolate and butter in a heatproof bowl set over a pan of barely simmering water. Stir until smooth, then remove the pan from the heat and leave the mixture to cool for 3–4 minutes.
Whisk the eggs and sugar in the bowl of a stand mixer fitted with the whisk, on medium speed for about 2 minutes, until well aerated and paler in colour. Add the melted chocolate mixture and vanilla and mix again until just combined.
Sift the flour, cocoa powder and salt into the bowl and fold them in using a rubber spatula until the mixture is smooth. Spoon the mixture into the prepared tin and spread it level. Bake the brownie on the middle shelf for 20 minutes, until slightly risen and set.
Meanwhile, make the cheesecake layer. In a mixing bowl, beat together the cream cheese and sugar until smooth. Add the vanilla and eggs and beat again to thoroughly combine. Add the soured cream and mix again until smooth.
In another bowl, mash the raspberries into a purée with a fork. Pass half of the mashed raspberries through a fine-mesh sieve to remove the seeds, then return the seedless purée to the bowl of remaining mashed raspberries and mix to combine.
Remove the brownie from the oven and tap the bottom of the tin sharply on the work surface to deflate slightly. Leave the brownie to cool for 3 minutes, then pour the cheesecake mixture over the top. Spoon the raspberry purée randomly over, then, using a wooden skewer, ripple the mixture through the cheesecake. Return the tin to the oven for a further 22–25 minutes, until the cheesecake sets.
Leave the brownie to cool to room temperature in the tin, then chill it for at least 1 hour before cutting it into squares or rectangles to serve (with extra raspberries, if you wish).