Line an 8×8-inch dish with parchment paper for easy removal later.
Combine chocolate chips, condensed milk, and butter in a saucepan over low heat until melted and silky.
Remove from heat and stir in vanilla extract to balance the flavor.
Pour mixture into the lined dish and smooth evenly with a spatula.
Warm raspberry jam slightly, add a drop of red food coloring if desired, and drizzle over fudge. Swirl lightly with a knife.
Refrigerate for at least 2 hours or until firm to the touch.
Lift out using parchment, cut into 36 squares, and serve chilled.
