Preheat oven to 350°F (175°C) and grease a 9×13-inch baking pan.
In a medium saucepan over low heat, melt the butter completely.
Stir in sugar until fully dissolved and smooth.
Remove from heat and whisk in the eggs one at a time.
Add flour, cocoa powder, and salt. Stir gently until just combined.
Stir in vanilla extract and chopped pecans.
Pour batter into the prepared pan and bake for 25–30 minutes, or until a toothpick comes out clean.
Sprinkle mini marshmallows evenly over the warm cake.
Broil for 1–2 minutes until the marshmallows are puffed and golden brown.
Let the cake cool slightly before slicing. Serve warm or at room temperature.
