In a medium bowl, beat the cream cheese and sugar until smooth. Add the egg, lemon juice, and vanilla extract, beating until well-blended. Stir in the hazelnuts.

Cover the bowl and refrigerate while you make the brownies.

Grease and flour a 9-inch square baking pan. Set aside.

Melt the chocolate and butter in a small saucepan over low heat, stirring frequently. Remove from heat to cool.

Combine the flour and salt. Set aside.

Using an electric mixer on medium speed, beat the sugar, eggs, and vanilla until smooth. Add the cooled chocolate mixture, and beat until well-combined.

Gradually add the flour mixture, mixing just until combined. Stir in the hazelnuts.

Spread half of the brownie batter into the prepared pan, just covering the bottom. Spread the chilled filling over the brownie batter. Refrigerate for 10 minutes.

Spread the remaining brownie batter on top of the filling. (Warning: This is far easier said than done. Take your time!) Swirl the layers by inserting a butter knife into the pan and lifting straight up. Repeat in a pattern or randomly over the surface of brownies.

Bake 30-33 minutes, or until a wooden pick inserted in the center comes out clean or with dry crumbs. Cool in the pan on a wire rack for 1 hour. Cut the brownies just before serving.