Line an 8-inch or 9-inch square pan with parchment paper or foil, leaving enough overhang on two sides to lift out the finished fudge. Lightly butter the lining.

Place the granulated sugar, unsalted butter, evaporated milk, and optional salt in a heavy saucepan. Heat over medium heat, stirring until the butter melts and the sugar dissolves.

Bring the mixture to a full rolling boil and boil for about 5 minutes, stirring frequently. If using a candy thermometer, cook to approximately 234 to 238°F, the soft-ball stage.

Remove the saucepan from the heat immediately. Stir in the semisweet chocolate chips until they melt completely and the mixture looks smooth.

Stir in the marshmallow creme, Irish cream liqueur, and vanilla extract until evenly blended. Fold in the chopped walnuts or pecans if using.

Pour the fudge into the prepared pan and smooth the top. Let it cool at room temperature, then refrigerate for 2 to 4 hours, or until firm.

Use the parchment or foil overhang to lift the fudge from the pan. Cut it into approximately 36 one-inch squares with a sharp knife.