Combine sugar, butter, evaporated milk, cream of tartar, and salt in a heavy 3 quart saucepan. Bring to a full boil over high heat, stirring constantly.
2½ cup white granulated sugar, ¾ cup unsalted butter, ⅔ cup evaporated milk, ½ teaspoon salt, ¼ teaspoon cream of tartar
Reduce heat to medium and continue to cook and stir for 5 minutes. Remove saucepan from heat.
To prevent crystallization of the sugar, scrape the sides of the pan with a water-dipped silicone spatula.
Stir in the peanut butter and stir until melted. Add Vanilla extract and marshmallow fluff, beating until both are well blended and the fluff leaves to tendrils.
1½ cup Peanut Butter, 7 oz jar of Marshmallow Fluff, 1 teaspoon vanilla extract
Fold in the M&M Peanut Candies (or use Reese's Pieces Candies). Spread fudge in the prepared 9 x 13 baking pan.
1½ cups Halloween M&M's or Reese's Pieces candies
Cool at room temperature for 2 to 3 hours until set up. Or, cool for 1 - 2 hours in the refrigerator. Slice when cool. Store in a plastic container.