Preheat the oven to 350°F. Spray two half sheet pans with non-stick baking spray with flour, line the pans with parchment paper, and spray the parchment with the baking spray. Set the pans aside until ready to use.
In a medium size bowl whisk together flour, baking powder, baking soda, and salt. Set aside until ready to use.
Place the unsalted butter in a large mixing bowl and beat until smooth. Then, add the granulated sugar and beat until light and fluffy.
Add the eggs, one at a time, beating well after each addition. Scrape the bowl, then add the sour cream and vanilla extract, and beat until well combined.
Add the flour mixture, about a cup at a time, beating until just combined after each addition. Scrape the bowl and add the buttermilk, mix until just combined.
Use a large ice cream scoop to divide the cake batter evenly between the two prepared pans. Use an offset spatula to smooth the batter into an even layer. Bake in a 350°F oven for 15-17 minutes.
Remove from the oven and allow to cool completely in the pans.
Once the cakes have cooled completely, remove them from the pans. Place a piece of parchment on top of the cake, then, place a wire cooling rack on top of the parchment. Invert the pan with the cooling rack. Remove the parchment from the bottom of the cake if it is still stuck to it from baking, place a clean piece of parchment on the cake, then place another wire cooling rack on top of the parchment, and invert both racks together so that the cake is right side up. Repeat with the other cake. Set the cakes aside while preparing the filling.