Preheat the oven to 325°F. Line a 9x9-inch square baking pan with parchment paper, leaving a 2-inch overhang on each side.

Make the brownie batter by combining chocolate and butter in a saucepan over low heat, stirring constantly until melted and smooth, or melt in the microwave at 50% power in short bursts.

In a large bowl, combine eggs, sugar, vanilla, and salt. Whisk or beat until pale and thick. Whisk in the warm melted chocolate mixture until smooth and glossy.

Sprinkle flour over the surface of the mixture and fold it in. Reserve about ⅓ cup of brownie batter and spread the rest evenly into the prepared pan.

Make the cheesecake batter by beating cream cheese in a medium bowl until smooth. Add sugar and vanilla, then beat until smooth and glossy. Beat in egg until well incorporated.

Spread the cheesecake batter evenly over the brownie batter in the pan. Spoon teaspoons of the reserved brownie batter randomly over the top.

Make the raspberry swirl by combining raspberries, sugar, and corn starch in a small bowl and mashing with a fork, leaving it slightly chunky. Spoon teaspoons of this mixture over the batter in the pan and swirl together with a skewer.

Bake for 30-35 minutes until puffed and the cheesecake is set in the middle with lightly golden edges.

Transfer the pan to a wire rack and let cool completely. Refrigerate for at least 1 hour before slicing with a hot dry knife.