Step 1: Bake the Chocolate Cake

1. Preheat oven to 350°F (175°C). Grease and flour a 9×13-inch baking dish.
2. In a large bowl, whisk together melted butter, sugar, eggs, and vanilla until smooth.
3. In a separate bowl, sift together flour, cocoa powder, baking powder, and salt.
4. Gradually add the dry ingredients to the wet mixture, stirring until fully combined.
5. Fold in chopped pecans or walnuts (if using).
6. Pour the batter into the prepared baking dish and spread evenly.
7. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Step 2: Add the Marshmallows

8. Remove the cake from the oven and immediately sprinkle the mini marshmallows over the top.
9. Return to the oven for 2-3 minutes, until the marshmallows are slightly melted and puffed.

Step 3: Prepare the Chocolate Frosting

10. In a saucepan over medium heat, melt butter and whisk in cocoa powder and milk until smooth.
11. Remove from heat and gradually whisk in powdered sugar and vanilla extract until creamy.

Step 4: Frost & Serve

12. Pour the warm chocolate frosting evenly over the marshmallow layer.
13. Let the cake cool for 15-20 minutes before slicing.
14. Serve warm or at room temperature for the best texture!