Preheat an oven to 180c (350f) and line a 9 inch pan with parchment paper. Butter the cake pan with room temperature butter to prevent the cake from sticking to the pan.
Into a medium-sized saucepan, over medium heat, add the brown sugar, kosher salt, butter, and corn syrup. Mix them together using a wooden spoon until the mixture is almost completely dissolved.
Add the chopped pecan and stir until all the pecans are covered with the caramel. Set the pecan mixture aside to cool and make the vanilla cake.
In a large mixing bowl, add the flour, baking powder, and baking soda, and mix with a whisk until combined. Set aside.
In a big mixing bowl, add the softened butter, sugar, brown sugar, and vanilla extract, and mix with an electric hand mixer for 2-3 minutes. Add the eggs and mix until well combined.
Add ½ of the dry ingredient mixture and ½ of the buttermilk into the butter mixture and mix with an electric hand mixer until just combined.
Add the remaining dry ingredients and the buttermilk and mix until just combined.
Pour the cooled pecan mixture into the prepared buttered baking pan and spread it with a spoon.
Pour the vanilla cake batter onto the pecan mixture and spread it with an offset spatula. Bake in a 180c (350f) oven for about 35-40 minutes or until a toothpick inserted into the cake comes out clean.
Allow the cake to cool to room temperature before flipping the cake. Run a knife around the edges of the cake, and top the pan with a plate. Flip the cake and tap the cake pan to release the cake from the pan. Enjoy!