Heat a wide skillet or pan (at least 1 inch deep) over medium to medium-high heat.
Test the pan by dropping a pinch of quinoa in; it should pop within a few seconds. Discard the test grain.
Sprinkle a few tablespoons of uncooked quinoa in a single layer in the hot pan. When it begins to pop, lift from the heat and shake or swirl the pan to agitate the grains.
Continue shaking off the heat until popping nearly stops, then pour the puffed quinoa into a bowl or rimmed baking sheet to avoid burning. Repeat in batches until all quinoa is puffed.
Melt the chopped chocolate in a heatproof bowl set over a saucepan of simmering water (bowl bottom should not touch the water), or microwave in 20-second bursts, stirring between each, until mostly melted. Remove and stir until smooth.
Fold the puffed quinoa into the melted chocolate until evenly coated.
For shaped pieces: coat a cookie cutter with cooking spray and set on a parchment-lined baking sheet, then fill with the chocolate-quinoa mixture and sprinkle with flaky salt if using. Alternatively, spread the mixture in a single even layer on the prepared baking sheet.
Refrigerate the baking sheet for 30 minutes, then remove chocolate from cutters or break the sheet into pieces of desired size. Serve immediately.