Line an 8x8-inch pan with parchment paper, leaving an overhang on two sides.
Toast the pecans in a dry skillet over medium heat for 3 to 5 minutes, stirring often, until fragrant. Let cool.
In a medium saucepan over low heat, combine the semi-sweet chips, milk chocolate chips, sweetened condensed milk and butter. Stir constantly until melted and smooth, about 5 minutes.
Remove from heat and stir in the vanilla and salt.
Let the mixture cool for 5 to 7 minutes, until warm but not hot.
Gently fold in the mini marshmallows and toasted pecans.
Spread into the prepared pan and press flat. Press extra marshmallows and pecans into the top.
Refrigerate for at least 3 hours, until firm.
Lift out by the parchment and cut into 36 squares, wiping the knife clean between cuts.