Preheat and Prep: Preheat your oven to 350°F (175°C). Generously grease a 9x13-inch baking pan with butter or non-stick baking spray. Set aside.
Combine Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking soda, and sea salt until evenly distributed.
Boil the Cola Mixture: In a medium saucepan over medium heat, add 1 cup of unsalted butter, 1 cup of Coca-Cola, and ¼ cup of cocoa powder. Whisk frequently until the butter is melted and the mixture comes to a gentle boil. Immediately remove from the heat.
Mix the Batter: Carefully pour the hot cola mixture over the dry ingredients. Stir with a spatula until just combined.
Add the Wet Ingredients: In a small bowl, whisk the buttermilk, beaten eggs, and vanilla extract. Add this to the batter and gently fold until incorporated. Do not overmix. Gently fold in the miniature marshmallows.
Bake: Pour the batter into the prepared baking pan. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Prepare the Frosting (Start this with 10 minutes left on the cake's bake time): In a clean saucepan, combine ½ cup of butter, ⅓ cup of Coca-Cola, and ¼ cup of cocoa powder. Bring to a simmer over medium heat. Remove from heat. Gradually whisk in the sifted powdered sugar and vanilla extract until completely smooth and glossy. Fold in chopped pecans if using.
Pour and Seal: As soon as you remove the hot cake from the oven, immediately pour the warm frosting evenly over the top. Use an offset spatula to gently spread it to the edges.
Cool: Allow the cake to cool completely in the pan at room temperature. The frosting will set and form a slight crust as it cools. Slice and serve!