For the cake: Preheat the oven to 350 degrees F. Spray a 9-by-13-inch baking pan with nonstick spray.
Combine the granulated sugar and butter in a stand mixer fitted with the paddle attachment on medium speed until blended. Add the eggs, vanilla and buttermilk and beat to combine. Whisk together the flour, baking powder and salt in a separate bowl. With the mixer on low, add the flour mixture to the wet mixture in three parts, mixing well between each addition and scraping down the bowl to fully combine. Put the dough in the prepared baking pan and press into an even layer across the bottom.
For the gooey: Beat together the cream cheese and butter in the cleaned mixer bowl until combined. Using the low stir setting, add the confectioners' sugar and eggs in three parts, alternating between the two and scraping down the sides of the bowl as needed, until combined. Mix in the vanilla and buttermilk. Pour the mixture over the dough in the pan and use a spatula to smooth over the top.
Bake until the cake has set and the edges are golden brown, 50 to 55 minutes. Cool for 15 minutes, then dust with more confectioners' sugar before serving.