Step 1: Line an 8x8 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal. Lightly grease the parchment paper.
Step 2: In a large microwave-safe bowl, combine the white chocolate chips, sweetened condensed milk, and butter.
Step 3: Microwave on high for 60 seconds. Stir well. Continue to microwave in 30-second intervals, stirring thoroughly after each, until the mixture is smooth and completely melted. This usually takes 2-3 minutes total.
Step 4: Stir in the vanilla extract until fully combined.
Step 5: Pour the white chocolate fudge mixture into the prepared baking pan and spread evenly with a spatula.
Step 6: Drizzle the red gel food coloring across the surface of the fudge. Start with 1/2 teaspoon and add more if a deeper color is desired.
Step 7: Using a skewer, toothpick, or the tip of a butter knife, gently swirl the red coloring into the white fudge to create a 'bloody' marbled effect. Be careful not to overmix.
Step 8: Refrigerate the fudge for at least 2 hours, or until completely set and firm.
Step 9: Once firm, use the parchment paper overhang to lift the fudge from the pan. Place it on a cutting board and cut into 1-inch squares. Serve and enjoy your spooky treat!