Preheat your oven to 350F.
Grease a baking pan.
Grab a bowl and mix the sugar, vanilla, and butter until it reaches a creamy texture. One at a time, slowly add the eggs. Do not add the next, until the previous is mixed in.
In a separate bowl, stir the cocoa, baking powder, salt, and flour. Mix well and then add into the butter mixture.
Add the pecans and mix well.
Pour the mixture into your greased pan.
Bake for around 15-18 minutes. They should be springy in the center.
Now place the marshmallows over the top and bake until marshmallows have melted.
Remove from oven and spread the marshmallows.
Now it's time to add the frosting (we'll be taking a look at how to make that next).
Add the frosting and your cake is ready to be cut into squares!