Line a square baking dish with parchment paper to make removing the chewies easier later.
Chop the dark chocolate into small, even pieces to ensure it melts smoothly and quickly.
In a medium saucepan over low heat, melt the butter and chopped chocolate together, stirring constantly until the mixture is glossy and smooth.
Remove the saucepan from heat and stir in the sweetened condensed milk and vanilla extract until well combined and silky.
Add the rolled oats and cocoa powder to the chocolate mixture. Mix vigorously with a spatula until all the oats are coated and the mixture is thick and fudgy.
Pour the mixture into the prepared dish, using your spatula to spread it evenly. Press down firmly to compact it for a dense, chewy texture.
Place the dish in the fridge and chill for at least 2 hours, or until the mixture is firm and set.
Once chilled, lift the block out of the dish using the parchment paper and transfer to a cutting board. Slice into 12 equal squares with a sharp knife, wiping the blade with a damp cloth between cuts for clean slices.
Serve immediately for a chewy, fudgy treat, or store in an airtight container in the refrigerator for up to a week.
