1. Preheat the oven to 350.
2. Mix flour, sugar, cocoa, baking soda, baking powder, and salt in a large bowl. Cut butter into slices and put in bowl with flour mixture. Set aside.
3. Bring oil and water just to a boil and then pour into dry ingredients. Mix until butter is completely melted.
4. Add buttermilk, eggs, and vanilla. Mix well.
5. Pour cake mixture into a greased 9x13 cake pan and bake for 20-30 minutes or until the edges start to pull away from the pan and a toothpick inserted in the middle comes out clean.
6. After the cake has cooled about 10 min. start preparing the frosting. Bring sugar, butter, and milk to a boil and boil for 20 seconds (Do NOT over boil! seriously, have a timer ready and as soon as the mixture boils start the timer and as soon as the timer goes off take the mixture off the stove).
7. Add chocolate chips. Stir until they're completely melted then stir in the mini marshmallows.
8. It is okay to frost the cake while it is still warm. We like to chill the cake once it is frosted and eat it cold!