Line an 8×8-inch baking dish with parchment paper, leaving overhang for easy removal.
In a medium saucepan, melt dark chocolate chips, condensed milk, and butter over low heat, stirring constantly.
Remove from heat, stir in vanilla, and add black gel coloring if desired.
Pour mixture into the prepared pan, smoothing the surface.
Melt white chocolate chips in the microwave, drizzle over the fudge, and swirl gently with a knife or skewer.
Dust the top with silver glitter while still warm.
Refrigerate for at least 2 hours, until firm.
Lift from the pan, cut into 36 squares, and serve.
