1. Preheat oven to 350°F and grease a 9×13 inch baking pan.

2. In a large bowl, whisk together flour, cocoa powder, sugar, baking soda, baking powder, and salt.

3. In a separate bowl, mix buttermilk, eggs, oil, and vanilla extract.

4. Combine wet and dry ingredients until just mixed.

5. Pour batter into pan and bake for 28–32 minutes, or until a toothpick comes out clean. Let cool completely.

6. In a mixing bowl, beat softened butter until smooth.

7. Add marshmallow fluff, powdered sugar, and vanilla extract. Beat until fluffy.

8. Spread filling evenly over cooled cake and refrigerate to set.

9. Heat heavy cream in a saucepan until it begins to simmer.

10. Pour cream over chocolate chips, let sit 1 minute, then stir until smooth.

11. Allow ganache to cool slightly, then pour over filling layer.

12. Spread ganache to edges and chill until set.

13. Let sit at room temperature 10 minutes before slicing and serving.