Preheat oven to 350°F. Butter and flour an 18x12 sheet pan and set aside.
In the bowl of your mixer, beat butter and sugar on medium speed until fluffy, about 2 minutes. Add eggs, one at a time, beating well after each. Beat in vanilla.
In a small bowl, combine flour, cocoa powder, baking soda, and salt. With the mixer on medium speed, gradually add to the butter mixture alternately with the milk, starting and ending with the flour mixture and beating until just barely combined after each addition. Fold in melted chocolate and food coloring until combined.
Spoon batter into prepared pan and bake for 17-20 minutes, until the top springs back when lightly touched in the center.
Spoon marshmallow fluff over the top of the warm cake and let it melt while you prepare chocolate frosting.
Melt butter in a small saucepan over medium heat.
Whisk in cocoa powder and milk, stirring until mixture is thickened and begins to boil. Remove from heat and whisk in powdered sugar until smooth.
Pour frosting over the cake, then swirl the marshmallow and chocolate frosting together with a spatula.
Sprinkle with marshmallows and pecans and let cool.