Line a baking sheet with parchment paper and set aside.
Place the dark chocolate chips in a microwave-safe bowl. Microwave on medium power for 30 seconds, then stir well. Repeat in 30-second intervals until the chocolate is almost completely melted but still has some small unmelted pieces (about 2-3 minutes total). Do not overheat—the residual heat will finish melting them.
Stir in the butter and vanilla extract until smooth and glossy. The mixture should look shiny, with no visible lumps of butter remaining.
Drop tablespoon-sized portions onto the prepared baking sheet, spacing them about 2 inches apart. Use a spoon to gently flatten each mound slightly—it helps them spread evenly as they cool.
Sprinkle sea salt over each chocolate snap while still warm. If using nuts, scatter them on top now too. Press lightly so they adhere.
Let the snaps sit at room temperature for about 20 minutes until firm to the touch but not completely hardened—they should have a slight give when pressed gently with your finger.