Stir the 1 teaspoon baking soda into the ½ cup buttermilk.
In a small saucepan, combine the 1 cup unsalted butter, the 3 tablespoons cocoa powder and 11 cup regular cola. Bring to a boil; once boiling, remove from heat.
Add in the 2 large eggs, one at a time, followed by the 2 cups granulated sugar, 2 cups all-purpose flour, ¼ teaspoon salt and v1 teaspoon vanilla extract. Stir in the buttermilk mixture.
Pour batter into prepared cake pan and bake at 350°F for 45 minutes (for the 9"x13" pan) or 1 hour and 15 minutes (for the Bundt pan). Cool cake 15-20 minutes in the Bundt pan prior to removing. Cool cake completely before frosting.
For the frosting
Bring the 4 tablespoons unsalted butter, 6 Tablespoons regular cola and 3 teaspoons cocoa powder to a boil in a small saucepan. Once boiling, remove from heat and whisk in sifted 3 ½ cups powdered sugar.
Pour immediately over cake and spread frosting as desired. Allow frosting to set before cutting cake.
Notes
If making the 9”x 13” version, the frosting layer will be thicker
Allow the frosting to set before serving
Store in an airtight container up to 3 days or freeze up to 3 months