Line a baking sheet with parchment paper.

Melt the dark or milk chocolate in 30-second microwave bursts, stirring between each, until smooth and glossy, about 90 seconds total.

Pour the melted chocolate onto the parchment and spread into a thin, even layer, about 1/4 inch thick.

Melt white chocolate the same way, then drizzle it over the dark chocolate layer.

Drag a toothpick through both chocolates to swirl them together.

Scatter chopped sour candy and crushed pretzels evenly over the top while the chocolate is still wet, pressing lightly so they stick.

Sprinkle with flaky sea salt.

Chill 20 minutes, until the chocolate is firm enough to snap cleanly when you press a corner.

Break into irregular shards to serve.