Preheat the oven to 350 degrees F. Line a 13x9 inch baking pan with parchment paper.
In a bowl combine the flour, baking powder and sugar.
In the bowl of a stand mixer add the eggs, sour cream, lemon zest, lemon juice and vegetable oil.
With a whisk attachment, beat the wet ingredients on medium-high until well combined and it becomes thicker and has more volume, about 5 minutes.
Scrape down the sides of the bowl a few times and then add the flour mixture on low, scraping down the bowl a few times.
Transfer the cake batter to the pan and evenly spread it out.
Place in the oven and bake for 40-45 minutes and then let cool.
To make the buttercream, add all of the ingredients to the bowl of the stand mixer and with a whisk attachment beat on low speed until the powdered sugar has been mixed in.
Turn the speed up to high and beat until soft and creamy.
Once the cake is cool, spread the buttercream on top and cut into individual squares.