Prepare Lemon Curd.
Beat cake mix and next 4 ingredients in a large bowl at low speed with an electric mixer until combined; increase speed to medium, and beat 2 minutes. Pour batter into 2 greased and floured 9-inch round cakepans. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on a wire rack 10 minutes; remove from pans, and cool completely on wire rack.
Beat whipping cream at high speed with an electric mixer until foamy; gradually add 3 tablespoons sugar, beating until soft peaks form. Gradually add sour cream, beating until mixture is spreading consistency. Chill at least 30 minutes.
Combine hot water, 1/3 cup sugar, and lemon juice in a small bowl, stirring until sugar is dissolved.
Place 1 cake layer on a cake plate; use a wooden pick to poke holes in cake 1/2-inch apart. Slowly pour half of sugar syrup mixture over cake layer until liquid is absorbed. Spread 1 cup Lemon Curd over layer. Top with remaining cake layer; use a wooden pick to poke holes in cake 1/2-inch apart. Pour remaining syrup over layer, and top with remaining Lemon Curd, leaving a 1 1/2-inch border around edge of cake. Spread sour cream frosting on sides of cake, and on 1 1/2-inch border around top edge of cake, leaving Lemon Curd visible. Chill at least 1 hour before serving. Store in refrigerator.
*For testing purposes only, we used Betty Crocker SuperMoist Cake Mix.
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