Chocolate Cake Batter: Preheat the oven to 350°F. Generously butter a metal 9x13 pan, making sure every crevice is well coated. Set aside.
In a medium saucepan over low heat, combine the butter, cocoa powder, salt, and water. Heat just until the butter is melted, stirring to combine. Remove from the heat and set aside to cool slightly.
In a large mixing bowl, whisk together the flour, sugar, baking soda, and baking powder until combined.
Pour the warm chocolate mixture into the dry ingredients and stir until fully combined.
Add the buttermilk, eggs, and vanilla extract. Mix until the batter is smooth and fully combined.
Pour the batter evenly into the prepared pan. Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool on a wire rack. Cool completely before adding the chocolate buttercream frosting.
Chocolate Buttercream Frosting: In a mixer cream the softened butter until smooth. Slowly add the powdered sugar and cocoa powder, mixing on low speed. Add the vanilla extract and heavy cream, then increase the speed to medium-high and whip until light and fluffy.
Frost the cooled cake. Enjoy!