Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large bowl, combine sugar, flour, baking soda, and salt.
Add eggs, buttermilk, and vanilla extract to the dry ingredients, and mix until smooth.
Pour the batter into the prepared dish and spread evenly.
Bake for 30–35 minutes or until a toothpick inserted in the center comes out clean.
While the cake bakes, combine pineapple (with juice) and chopped pecans in a saucepan.
Bring to a boil, then reduce heat and simmer for 5 minutes. Set aside to cool slightly.
Spread the warm pineapple mixture evenly over the hot cake.
For the frosting, beat together butter, cream cheese, powdered sugar, and vanilla until fluffy.
Once the cake has cooled completely, spread the frosting over the pineapple layer.
Chill for at least 1 hour before serving.