Line a shallow 8-inch square pan with parchment, leaving an overhang for lifting the cooled fudge.
Place the sugar, evaporated milk, butter, and salt in a saucepan. Heat over medium heat, stirring across the bottom of the pan.
When the mixture reaches a steady boil, start a five-minute timer. Keep stirring continuously for the full five minutes and adjust the heat to prevent overflow.
Add the semisweet chocolate chips and stir until they have melted and the mixture is smooth.
Take the saucepan off the burner before adding the remaining ingredients.
Stir in the mini marshmallows, vanilla extract, and chopped pecans. Continue mixing until the marshmallows melt completely and no white pockets remain.
Spread the warm mixture evenly in the prepared pan, working it into the corners.
Cool until the fudge is firm enough to hold its shape when cut. Chill after cooling if needed; the setting time can vary.
Lift the firm fudge onto a cutting board and cut it into 25 small squares, making five rows in each direction. Wipe the knife between cuts.