Line your baking pan with parchment paper, leaving some overhang for easy lifting.
In a microwave-safe bowl, melt the butter in short bursts, about 30-40 seconds, until it’s just melted and fragrant.
Add the honey or maple syrup, cocoa powder, and vanilla extract to the melted butter. Stir vigorously with a spatula until the mixture is smooth, glossy, and smells rich and chocolatey.
Pour the chocolate mixture over the rolled oats in a large bowl. Use the spatula to fold everything together until the oats are evenly coated and sticky.
Transfer the mixture into your prepared pan. Use the back of a spatula or your fingers to press it down firmly, creating a compact, even layer.
Place the pan in the fridge and chill for at least 2 hours, or until the slice is firm and set. If in a hurry, pop it in the freezer for about an hour.
Once set, lift the slice out of the pan using the parchment overhang. Place it on a cutting board.
Use a sharp knife to cut the slice into 12 even squares. Press down firmly to get clean cuts, and wipe the knife between slices if needed.
Serve the slices immediately or store them in an airtight container in the fridge for up to a week. For longer storage, freeze the slices and thaw slightly before enjoying.
