In a medium bowl, beat cream cheese and sugar until smooth. Add egg, lemon juice, and vanilla extract, beating until well-blended. Stir in hazelnuts. Cover bowl and refrigerate.
Grease and flour a 9-inch square baking pan. Set aside.
Melt chocolate and butter in a small saucepan over low heat, stirring frequently. Remove from heat to cool.
Combine flour and salt. Set aside.
Beat sugar, eggs, and vanilla until smooth. Add cooled chocolate mixture. Beat until well-combined. Gradually add flour mixture, mixing just until combined. Stir in hazelnuts.
Spread half of the brownie batter into prepared pan, just covering the bottom. Spread chilled filling over brownie batter. Refrigerate for 10 minutes.
Preheat oven to 350°.
Spread remaining brownie batter on top of the filling. (Warning: This is far easier said than done.) Swirl the layers by inserting a butter knife into the pan and lifting straight up. Repeat in a pattern or randomly over 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 pan on wire rack for 1 hour. Cut brownies just before serving.
Brownies should be covered and stored in the refrigerator.
Makes 12-18 brownies.