In a large, heavy saucepan or Dutch oven, combine butter, canned milk, and sugar.

Stir over medium heat until the sugar dissolves completely.

Bring the mixture to a boil, then cover and boil for 5 minutes.

Turn off the heat source.

Add marshmallows and stir until they are completely melted and incorporated.

Add the unsweetened chocolate, stirring until melted and smooth.

Add the semi-sweet chocolate chips, stirring until melted and smooth.

Add the German sweet chocolate, stirring until melted and smooth.

Stir in the vanilla extract.

Incorporate the chopped pecans or walnuts, blending evenly throughout the fudge.

Pour the fudge mixture into a buttered 10 x 15-inch pan.

Allow the fudge to stand at room temperature until firm and set.

Cut the fudge into 1-inch squares.

If desired, press whole pecan halves into the tops of the squares for decoration.