Whisk together thoroughly: flour, salt, and baking soda.

Melt, stirring constantly, in a large, heavy saucepan over very low heat: bittersweet chocolate and 4 tablespoons (1/2 stick) unsalted butter.

Let cool until barely warm.

Add 2/3 cup sugar and 2 ½ teaspoons vanilla and beat with a wooden spoon until well blended.

Beat in 2 large eggs one at a time until well combined.

Stir in 2 tablespoons corn syrup.

Stir the flour mixture into the chocolate mixture until well blended and smooth. Scrape the batter into the pan and spread to the edges. Bake for 12 minutes.

Meanwhile, combine in a food processor until well blended: cream cheese, ½ cup sugar, 2 teaspoons unsalted butter — melted, 1 large egg, 1 teaspoon vanilla, ¼ teaspoon finely grated lemon zest.

Spread the cream cheese topping evenly over the chocolate layer. Reduce the oven temperature to 325’. Return the pan to the oven. Bake until the cheesecake layer is just tinged with brown and beginning to crack on top and a toothpick inserted in the center comes out clean, but still moist and fudgy at the bottom, 32 to 36 minutes. Remove the pan to a rack and let stand until completely cool. Refrigerate until well chilled before slicing. Using the overhanging foil as handles, life the brownies to a cutting board. Carefully peel off the foil. Cut into bars.