Line an 8-inch square pan with parchment, leaving overhang for lifting.
Chop the cranberries and nuts, if using. Pat cranberries dry with a paper towel.
Combine white chocolate, condensed milk, and butter in a heavy saucepan.
Warm over low heat, stirring constantly, until almost melted and smooth.
Remove from heat. Stir in vanilla, salt, and orange zest until glossy.
Fold in most cranberries and nuts, keeping a little for the top.
Spread fudge into the pan. Tap gently to level and release air.
Scatter reserved cranberries and nuts over the surface. Press lightly to adhere.
Chill until firm, about 2 to 3 hours. Lift out and cut into squares.