Chop the dark chocolate into small, even pieces and set aside. This helps it melt quickly and smoothly.
Place the chopped chocolate in a heatproof bowl and microwave in 30-second bursts, stirring well after each. Continue until the chocolate is glossy and fully melted, about 1-2 minutes.
Measure out the honey or maple syrup and add it to the melted chocolate, stirring until well combined. The mixture will become sticky and smooth.
Stir in the melted butter until the mixture is shiny and uniform. This adds richness and helps everything come together.
Pour the warm chocolate mixture into a large bowl and add the rolled oats. Mix thoroughly with a spatula until all the oats are coated and the mixture is thick and sticky.
If desired, fold in chopped nuts or dried fruit for added texture and flavor. Distribute evenly throughout the mixture.
Line the baking dish with parchment paper and transfer the mixture into it. Press firmly with the back of a spatula to flatten and compact the mixture evenly.
Let the mixture sit at room temperature for about 30 minutes, then refrigerate for at least 1 hour until firm and chewy.
Once set, lift the block out of the dish using the parchment paper and cut into squares with a sharp knife. For neat slices, warm the blade slightly or wipe it with a hot cloth.
Serve immediately or store in an airtight container in the fridge for up to 5 days. For longer storage, freeze the chewies wrapped tightly in plastic wrap and foil for up to 3 months.
