Preheat the oven to 350˚F (177˚C). Lightly grease a 9-inch round baking pan. Line the bottom with a parchment paper.
In a medium mixing bowl, whisk together flour, baking powder, salt and baking soda.
In a large mixing bowl with an electric mixer fitted with a paddle attachment, beat the butter until smooth. Add sugar and beat on medium speed for 3 minutes. Scrape down the sides of the bowl. Beat in eggs, one at a time, scraping down the side of bowl as needed. Add sour cream, lemon zest and lemon juice, beat for 1 minute. Add flour mixture and beat on medium speed for 25 seconds. The mixture is thick. Use a spatula to fully incorporate the flour if needed.
Transfer the batter into the prepared baking pan. Bake in the oven for 25 - 30 minutes, or until a toothpick inserted into the cake comes out with a few moist crumbs attached. Remove from oven and let the cake cool completely in the pan on a wire rack.
For the frosting, in a large mixing bowl with an electric mixer fitted with a paddle attachment, beat butter until smooth, about 3 minutes. Add lemon zest, lemon juice and salt. Mix again. Add powdered sugar and beat over high speed until smooth with no lumps. Add sour cream and vanilla, and beat for 1 more minute to combine.
To assemble, run a sharp knife around the edges of the cake pan and invert the cake into a plate or your hand. Remove the parchment paper. Place the cake on a cake stand or large plate. Spoon about ¾ of the frosting over the top of the cake. Add more frosting if needed. Gently smooth the top with a spatula and creat stripe pattern if desired. Serve.