Preheat your oven to 350°F (175°C).
Melt the butter and semi-sweet chocolate together using a double boiler or microwave. Set aside to cool slightly.
In a mixing bowl, combine the melted chocolate and butter with 1 cup of sugar. Whisk in the eggs, one at a time, then stir in the vanilla extract.
Sift together the flour and salt, then fold them into the chocolate mixture until just combined.
In another bowl, mix the ricotta cheese, remaining 1 cup of sugar, lemon zest (if using), and one egg until creamy and smooth.
Spread half of the brownie batter into the bottom of the prepared baking pan. Spoon the ricotta mixture over the brownie base, then dollop the remaining brownie batter on top and swirl together.
Bake for about 35-40 minutes, or until the edges are set and a toothpick comes out with a few moist crumbs.
Allow to cool in the pan for at least 30 minutes before slicing into squares.
