Line large baking sheet with parchment paper.
In a large pan over medium-high heat, toast sunflower seeds until lightly golden brown, about 1-2 minutes.
Into a stainless steel or glass bowl, break chocolate bar into small pieces, add coconut oil, and melt using double boiler method over low heat. Alternatively, melt chocolate and coconut oil in a microwave-safe bowl in microwave in 30-second increments until melted. No matter the method you choose, be very careful to not burn chocolate. If it looks mostly melted, remove from stovetop heat or do not return to microwave. Just keep stirring until all chocolate is melted. (See note below if your chocolate has burned.)
Pour toasted sunflower seeds into bowl of melted chocolate and stir to coat seeds. Using a spoon, scoop out ~1 tbsp of mixture at a time and transfer to prepared baking sheet until you have 12 clusters.
Transfer to refrigerator to chill until mostly solidified, about 15 minutes, before transferring to freezer to fully solidify, about an additional 5 minutes (you can choose to let them harden solely in the refrigerator, but it will take a little longer).
Peel parchment paper from back of clusters to remove.