Preheat oven to 325°F. Grease and flour a bundt pan and set it aside.
In a medium bowl, whisk together 3 cups flour, 1 teaspoon salt, 1 teaspoon baking powder, and 1/2 teaspoon baking soda. Set the dry mixture aside.
In a large mixing bowl, cream 1 cup softened butter with 2 cups sugar using a hand mixer until light and fluffy.
Add 4 eggs to the butter mixture one at a time, mixing just until combined after each egg.
Add the flour mixture and 1 cup buttermilk to the batter, alternating between them and beginning and ending with the flour mixture. Mix until just combined after each addition.
Spread the batter evenly into the prepared bundt pan. Bake 55–60 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and let it cool in the pan for 10–15 minutes.
While the cake is cooling, make the sauce: in a medium saucepan combine 1/3 cup butter, 3/4 cup sugar, 2 teaspoons vanilla, and 3 tablespoons water. Cook over medium–high heat, stirring occasionally, until the butter is melted and the sugar is dissolved. Do not boil.
Using a wooden skewer, poke holes all over the warm cake while it is still in the pan.
Pour the warm sauce evenly over the cake, allowing it to soak in. Let the cake cool completely in the pan so it absorbs the sauce.
When fully cooled, invert the cake onto a serving plate and serve.