Preheat the oven to 350º. Line a 9 x 13 pan with parchment paper and spray with cooking spray.

In the bowl of a stand mixer, sift together the flour, sugar, cocoa powder, baking soda, baking powder, and salt.

In a small bowl, whisk the eggs with the vegetable oil and vanilla and add to the dry mixture. Mix until combined.

In a measuring cup combine the buttermilk and sour cream and add to the mixture, mixing until well combined. Add boiling water and mix on low speed until combined.

If desired add stir in coconut and pecans and pour into the prepared pan.

Bake for 26-30 minutes until a toothpick comes out clean.

Remove from the oven and immediately cover the top with marshmallows.

Melt the butter in a medium bowl in the microwave. Add ⅔ cup cocoa powder and whisk until smooth. Add sifted powdered sugar and ½ cup cream. Whisk until smooth, add additional cream if needed. The frosting should be thin enough to spread.(If you don’t sift the powdered sugar, you can use a hand mixer to mix to remove any lumps)

While the cake is still warm, spread the frosting over the marshmallows. It will not be completely smooth. Let it cool on a wire rack before cutting.