In a small saucepan, combine raspberries, sugar, and lemon juice over medium heat. Cook for 3 to 5 minutes, stirring and gently smashing the berries, until they release juices and start to thicken. Stir in the cornstarch mixture and cook 30 to 60 seconds until glossy and thick. Remove from heat and let it cool to lukewarm.

Optional: Press through a fine sieve to remove seeds for a smooth swirl.

Brownie batter first: Melt butter and chopped chocolate together in a medium saucepan over low heat, stirring until smooth. Remove from heat, whisk in both sugars, then the egg, yolk, and vanilla. Sprinkle in the cocoa, flour, and salt, and fold until combined.

Cheesecake swirl next: In a separate bowl, whisk cream cheese until very smooth. Add sugar, sour cream, vanilla, and salt. Whisk until no lumps remain, then whisk in the egg just until blended.

Spread about two thirds of the brownie batter into your lined pan. Dollop spoonfuls of cheesecake mixture on top, leaving exposed brownie in between.

Add small spoonfuls of raspberry sauce over the cheesecake areas and bare brownie spots. Spoon the remaining brownie batter over any large gaps.

Using a skewer or butter knife, gently swirl the layers together with figure eight motions.

Bake in a 325°F (160°C) oven for 22 to 28 minutes. Edges should look set and slightly puffed, and center should have a slight jiggle.

Let the pan cool on a rack until barely warm, then chill for at least 1 hour for clean slices. Lift the slab out by the parchment and cut into 9 squares.