Prepare an 8x8-inch pan by lining it with parchment paper. Use a large enough sheet so that the paper comes up the sides and hangs slightly over the edge. This makes it easier to lift the fudge out of the pan.
In a large heat-safe mixing bowl, combine the semi-sweet chocolate, milk chocolate chips, marshmallows, and 1 ¾ cup chopped pecans together. Set this bowl aside for later.
In a medium-sized saucepan over medium-high heat, bring sugar, butter, and evaporated milk to a boil. Cook the boiling mixture, stirring continuously (this is important!) for 5 minutes. The mixture will darken slightly in color and become thicker.
Immediately pour the boiling mixture into the bowl with the chocolate, marshmallows, and pecans. Stir to combine, until the mixture is thick and elastic. *See NOTE 2 - You can either leave some marshmallows un-melted, or continue stirring until all of the chocolate and marshmallows are melted.
Scoop the chocolate marshmallow fudge mixture into the parchment-lined pan. Use an offset spatula to spread the mixture into the corners and ensure that the thickness is even.
Sprinkle on ¼ cup pecans over the top. Using the palm of your hand, gently press down on the pecans so that they are not sitting loosely on the top.
Place fudge in the refrigerator for 2 hours or until completely set.
Once set, cut into 16 one-inch cubes. Serve and enjoy!
Notes
I highly recommend preparing the chocolate, marshmallow and nut mixture before you begin melting your butter and sugar together. This will allow you to pour the boiling mixture directly into the chocolate and marshmallow mixture. The chocolate will melt much more quickly, resulting in less work for you.
You may choose to keep stirring sugar and chocolate mixture until all of the marshmallows are melted, but I prefer to leave my marshmallows less melted. It’s completely a matter of preference.