Preheat the oven to 350˚F (175˚C). Grease and flour an 8-inch round cake pan. Set aside.
In a large mixing bowl, whisk together the eggs, oil, vanilla extract, sour cream and water until well blended.
In another medium bowl, sift together the flour, sugar, baking powder and salt. Slowly add the dry ingredients to the wet ingredients and mix with a handheld electric mixer or stand mixer fitted with the paddle attachment on low speed for about 30 seconds until just combined.
Add in the lemon zest and cubed butter and continue to mix on low speed until everything is evenly incorporated into a thick batter; about 2 minutes of mixing time total. Do not overmix!
Pour the batter into the prepared pan and spread it evenly. Bake for 68-70 minutes, or until a cake tester inserted in the center comes out clean. Remove from the oven and let cool for about 15-20 minutes before carefully inverting onto a cooling rack. Cool completely before frosting.
For Buttercream:In a medium bowl, beat together the butter and confectioners’ sugar until creamy and light, about 1 minute of mixing time in total. Add a pinch of salt, vanilla extract, lemon juice (if desired), and hot water, and mix on low speed to combine. Increase the speed to medium-high and beat until light and fluffy; about 3 more minutes of mixing time total.Spread frosting over the top of the cooled cake. Slice and serve.