Preheat your oven to 350 degrees and generously grease two 8-inch cake pans. Set aside.
In a large bowl, beat together the butter and sugar until fluffy. Add in the eggs, one at a time, beating between each addition. Then add the sour cream.
Add in the flour, baking powder, and salt, and beat until combined. The batter will be very thick. Slowly beat in the lemon juice and lemon zest, and beat until a smooth batter forms. Pour evenly into your cake pans, and bake for 20-25 minutes, or until a cake tester comes out clean. Cool completely before frosting.
In a small saucepan, cook the flour, milk, salt, and lemon zest until a thick paste forms. You'll want to stir the mixture constantly while heating over medium heat. Remove from heat, and cool the mixture COMPLETELY before continuing.
In a large bowl or the bowl of a stand mixer, beat the butter and sugar for 5-10 minutes, or until the mixture is noticeably lighter in color, and the sugar has mostly dissolved.
Beat in the flour mixture, and beat until it becomes light and fluffy. It will separate during this process, just keep beating.
To frost the cake, place one cake on your plate, and pipe a ring of frosting around the edge to act as a barrier for the lemon curd. Spread the lemon curd in the middle of the cake evenly. Top with the second cake, and frost as desired. Enjoy!