Preheat oven to 350F. Line a 9x9-inch pan with parchment, leaving overhang for easy removal. Combine butter and chocolate in a saucepan over low heat, stirring until smooth. Remove from heat and let cool 5 minutes.

Whisk 1 cup sugar into the chocolate mixture until glossy. Add 2 eggs and vanilla, beating vigorously until the batter looks thick and pulls away from the sides of the bowl, about 1 minute.

Stir in flour, cocoa powder, and salt until just combined. Do not overmix. Spread all but 1/2 cup of batter into the prepared pan.

Beat cream cheese with remaining 1/2 cup sugar until fluffy, about 2 minutes. Add the remaining egg and 1/4 cup raspberry preserves, beating until smooth and no lumps remain.

Dollop cheesecake mixture over the brownie base. Add reserved brownie batter in spoonfuls. Use a knife to swirl both layers together, running it through 4-5 times for a marbled effect. Bake 30-35 minutes until the center is set but still slightly jiggly.

Warm the remaining 1 tablespoon preserves and drizzle over the hot brownies. Dot with fresh raspberries if using. Cool completely in the pan, at least 2 hours, then lift out using parchment and cut into squares.