Grease a 9x9-inch baking pan with butter and set it aside.
In a medium saucepan, combine the unsalted butter, evaporated milk, and white sugar.
Place the saucepan over medium heat.
Whisk frequently while the mixture heats.
Bring the mixture to a steady boil.
Continue boiling for 5 minutes, whisking often to help dissolve the sugar and prevent scorching.
Remove the saucepan from the heat.
Immediately add the semisweet chocolate chips and marshmallows.
Stir continuously until the chocolate chips and marshmallows are fully melted and the mixture is smooth.
Add the vanilla extract.
Stir until fully incorporated.
Add the chopped pecans.
Stir until the pecans are evenly distributed throughout the fudge mixture.
Pour the hot fudge mixture into the prepared baking pan.
Spread it evenly with a spatula.
Place the pan in the refrigerator.
Chill for at least 30 minutes, or until the fudge is fully set.
Once firm, remove the fudge from the refrigerator.
Cut into squares.
Serve as desired.