Preheat your oven to 350°F.

Spray an 18x13-inch jelly roll pan with non-stick cooking spray and set aside.

In a large mixing bowl, whisk together the flour, sugar, baking soda, and salt.

In a small bowl, whisk together the sour cream, eggs, and vanilla extract. Add this mixture to the dry ingredients and mix until just combined.

In a medium saucepan, combine water, butter, and cocoa powder. Bring to a boil over medium-high heat.

Pour the hot chocolate mixture into the cake batter and whisk until smooth.

Pour the batter into the prepared pan and spread evenly.

Bake for 15-18 minutes or until a toothpick inserted in the center comes out clean.

Add the milk, cocoa powder, and butter to a medium saucepan. Bring to a boil over medium-high heat.

Add the powdered sugar with the hot cocoa mixture. Remove from heat. Whisk until smooth. Use a hand mixer if necessary to remove lumps.

Pour half of the hot frosting over the warm cake and spread evenly.

Sprinkle the mini marshmallows and pecans over the frosting.

Drizzle the remaining frosting over the top, ensuring the marshmallows and pecans are well coated.

Let the cake cool completely before slicing and serving.