Preheat an oven to 180c (350f). Butter the bundt cake pan with room temperature butter and flour to prevent the cake from sticking to the pan.
In a big mixing bowl, add the room temperature butter, sugar, brown sugar, vanilla extract, and kosher salt, and mix with an electric hand mixer until it's creamy.
Add the room temperature eggs, sour cream, oil, and mix until combined.
Add ½ of the flour and the baking powder, and ½ of the buttermilk, and mix until almost combined.
Add the rest of the flour and the buttermilk and mix until just combined and there are no lumps of flour visible. Dont overmix!
In a small mixing bowl, add the granulated sugar and the cinnamon and mix well until combined.
Fill the pan halfway with the cake batter, then sprinkle an even layer of cinnamon sugar on. Top the cake with the remaining cake batter, and make swirls using a knife.
Bake in a 180c (350f) oven for about an hour or until a toothpick inserted into the cake comes out clean.
Allow the cake to cool to room temperature before flipping the cake. Top the pan with a plate, flip the cake and tap the cake pan to release the cake from the pan.