Preheat the oven to 325 degrees F. Line an 8 inch round springform pan with parchment paper and grease the sides.
For the brownie base, combine chocolate and butter in a heatproof bowl and melt gently in the microwave. Stir until smooth. Mix in sugar and egg until evenly incorporated and mixture is smooth and glossy. Mix in vanilla and salt, then gently stir in flour.
Spread the brownie mixture evenly into the prepared pan and bake for 10 minutes until just set.
For the cheesecake mixture, beat together cream cheese and sugar until smooth in a medium bowl. Beat in egg, then mix in yogurt, peanut butter and vanilla. Pour the batter over the brownie base and spread it out evenly.
For the topping, melt together chocolate and peanut butter and spoon it randomly over the cheesecake mixture. Swirl it in using a skewer.
Bake for 15-20 minutes until the center is just set with a slight wobble. Transfer to a wire rack and let cool completely, then refrigerate for at least 4 hours.