Line a large baking sheet (approximately 13x18 inches) with parchment paper, making sure it lies flat without wrinkles. Set aside.

Place the chocolate in a large microwave-safe bowl. Microwave in 30-second intervals, stirring thoroughly between each interval, until the chocolate is 90% melted with just a few small pieces remaining. Stir continuously until completely smooth — residual heat will melt the last bits.

If using vanilla extract, stir it into the melted chocolate until fully incorporated. Work quickly as the chocolate will begin to set.

Pour the melted chocolate onto the prepared baking sheet. Use an offset spatula to spread it into an even layer about 1/4-inch thick, creating a rough rectangle or oval shape.

Immediately scatter the rainbow sprinkles evenly over the wet chocolate surface, covering it completely. Gently press the sprinkles down with your fingertips or the back of a spoon so they adhere.

If using sea salt, sprinkle it lightly over the top for a sweet-salty contrast. Transfer the baking sheet to the freezer.

Freeze for 15-20 minutes until the bark is completely firm and set. Remove from the freezer and let it sit at room temperature for 2-3 minutes.

Lift the parchment paper with the bark onto a cutting board. Use your hands to break the bark into irregular shards of various sizes, typically 2-4 inches each.