Line your 8-inch square dish with parchment paper, leaving some overhang for easy removal.
Place the butter in a saucepan over low heat and gently melt it, stirring occasionally until it just shimmers and smells nutty.
Pour the melted butter into a large mixing bowl, then stir in the honey or maple syrup and vanilla extract until smooth and combined.
Add the rolled oats to the wet mixture and stir well to coat every grain evenly, creating a sticky, textured base.
Stir in the chopped dark chocolate and a pinch of sea salt, mixing until the chocolate is evenly distributed throughout the mixture.
If using, fold in nuts or dried fruit to add extra crunch or sweetness.
Transfer the mixture to the prepared dish and press it down firmly with the back of a spatula or your fingers, smoothing out the surface for an even layer.
Place the dish in the fridge and chill for at least 2 hours, or until the slice is firm and set. For quicker results, freeze for about an hour.
Once set, lift the slice out of the dish using the parchment paper overhang, and transfer to a cutting board.
Use a sharp knife dipped in hot water to cut the slice into squares or rectangles, wiping the blade between cuts for clean edges.
Serve your chocolate oat slice chilled or at room temperature, enjoying the chewy, fudgy texture with a burst of rich chocolate flavor.
