Preheat your oven to 350°F (175°C) to ensure even baking and proper rising.
Grease a 9x13 inch baking pan with butter or shortening; optionally line with parchment paper for easy removal.
In a saucepan over low heat, melt together 4 ounces butter, 1 cup Coca-Cola, and 1 ounce baking chocolate. Stir until smooth and remove from heat to cool slightly.
In a large mixing bowl, combine the melted chocolate mixture with shortening, vegetable oil, sugar, eggs, vanilla, and buttermilk. Use an electric mixer to blend well.
In another bowl, whisk together all-purpose flour, baking soda, baking powder, salt, and cocoa. Gradually add dry ingredients to the wet mixture, mixing until just combined.
Gently fold in 3/4 cup miniature marshmallows into the batter for a chewy surprise.
Pour batter into the prepared pan and spread evenly. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
Remove cake from oven and cool in pan for 10 minutes, then transfer to a wire rack to cool completely.