Puff the quinoa: Heat a wide skillet or pan that is at least 1-inch deep over medium to medium high heat. (Note: different pots will heat and puff the quinoa differently, so you may need to experiment with yours to find the right temperature so that the quinoa pops but does not burn—I find that non-stick surfaces do best over medium-high heat, while stainless steel works best over medium.)
Test your pot's temperature: Drop a small pinch of quinoa into the pan. It should pop within a few seconds. If this does not occur, adjust your pot's heat or wait a bit longer, until the test quinoa pops as described. Discard the "test" quinoa.
Sprinkle a few tablespoons of quinoa over the pan's surface. Add only enough to coat the pan in a single layer, without crowding or overlapping the quinoa grains. As soon as the quinoa begins to pop (it may happen immediately), lift from the heat and shake the pan to agitate the grains. Continue shaking the pan off the heat, then as soon as the popping sound almost stops, pour the grains into a bowl or rimmed baking sheet (if you leave them in the pot, they may burn). Repeat, toasting the quinoa in batches until you have made the desired amount. If the quinoa tastes or smells burned, it has been overheated or left in the pan too long. Discard, as it will cause your chocolate bark to taste bitter.
Melt the chocolate in a heatproof bowl set over a saucepan of simmering water (do not let the bottom of the bowl touch the water), or melt it in the microwave in 20-second bursts, stopping to stir the chocolate between each. When only a few small pieces of chocolate remain, remove from the heat (or microwave) and stir until smooth, letting the residual heat finish melting it completely. Fold in the puffed quinoa.
To make chocolate bark shapes: coat a cookie cutter of any shape with cooking spray, place it on a baking sheet lined with parchment paper or a silpat mat. Pour the chocolate and quinoa into the cookie cutter, then sprinkle with a pinch of salt, if using. Alternatively you can spread the chocolate in a single layer on a parchment or wax paper–lined baking sheet.
Place the baking sheet in the refrigerator for 30 minutes to allow the chocolate to set. Gently remove the chocolate from the cookie cutters or, if making a single sheet, break the chocolate into pieces of desired size. Enjoy immediately.
*Depending upon the type of quinoa you purchase, you made need to rinse it first to remove any bitter taste. I've found that quinoa sold in the US is usually pre-rinsed (sometimes the package will specify), and none of the brands I've tried have tasted bitter. Unless I were to buy quinoa from a bulk bin, I skip rinsing. If you do prefer to rinse your quinoa, it will need to dry completely before being puffed. You can accomplish this by spreading the rinsed quinoa on an ungreased baking sheet and baking it in a 100 degrees F oven for one hour, or by spreading it into a single layer and leaving it out to dry overnight.